WASM builds fail on bindings generated by bindgen 0.70.0 or later

[ajrcarey]

During import of pdfium_6666 bindings as part of #137, upgraded bindgen dependency to latest 0.70.1 and regenerated bindings using command

cargo build --features "bindings"

WASM builds fail to build with errors:

error[E0080]: evaluation of constant value failed
|    --> /pdfium/src/bindgen/pdfium_6666.rs:652:28
|     |
| 652 |     ["Size of FPDF_BSTR_"][::std::mem::size_of::<FPDF_BSTR_>() - 16usize];
|     |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempt to compute `8_usize - 16_usize`, which would overflow
| 
| error[E0080]: evaluation of constant value failed
|    --> /pdfium/src/bindgen/pdfium_6666.rs:809:38
|     |
| 809 |     ["Size of FPDF_LIBRARY_CONFIG_"][::std::mem::size_of::<FPDF_LIBRARY_CONFIG_>() - 48usize];
|     |                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempt to compute `24_usize - 48_usize`, which would overflow
| 
| error[E0080]: evaluation of constant value failed
|    --> /pdfium/src/bindgen/pdfium_6666.rs:882:33
|     |
| 882 |     ["Size of FPDF_FILEACCESS"][::std::mem::size_of::<FPDF_FILEACCESS>() - 24usize];
|     |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempt to compute `12_usize - 24_usize`, which would overflow
| 
| error[E0080]: evaluation of constant value failed
|    --> /pdfium/src/bindgen/pdfium_6666.rs:936:35
|     |
| 936 |     ["Size of FPDF_FILEHANDLER_"][::std::mem::size_of::<FPDF_FILEHANDLER_>() - 56usize];
|     |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempt to compute `28_usize - 56_usize`, which would overflow
| 
| error[E0080]: evaluation of constant value failed
|     --> /pdfium/src/bindgen/pdfium_6666.rs:1053:35
|      |
| 1053 |     ["Size of FPDF_COLORSCHEME_"][::std::mem::size_of::<FPDF_COLORSCHEME_>() - 32usize];
|      |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempt to compute `16_usize - 32_usize`, which would overflow
| 
| error[E0080]: evaluation of constant value failed
|     --> /pdfium/src/bindgen/pdfium_6666.rs:1624:20
|      |
| 1624 |     ["Size of tm"][::std::mem::size_of::<tm>() - 56usize];
|      |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempt to compute `44_usize - 56_usize`, which would overflow
| 
| error[E0080]: evaluation of constant value failed
|     --> /pdfium/src/bindgen/pdfium_6666.rs:1649:26
|      |
| 1649 |     ["Size of timespec"][::std::mem::size_of::<timespec>() - 16usize];
|      |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempt to compute `8_usize - 16_usize`, which would overflow
| 
| error[E0080]: evaluation of constant value failed
|     --> /pdfium/src/bindgen/pdfium_6666.rs:1667:28
|      |
| 1667 |     ["Size of itimerspec"][::std::mem::size_of::<itimerspec>() - 32usize];
|      |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempt to compute `16_usize - 32_usize`, which would overflow
| 
| error[E0080]: evaluation of constant value failed
|     --> /pdfium/src/bindgen/pdfium_6666.rs:1695:33
|      |
| 1695 |     ["Size of __locale_struct"][::std::mem::size_of::<__locale_struct>() - 232usize];
|      |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempt to compute `116_usize - 232_usize`, which would overflow
| 
| error[E0080]: evaluation of constant value failed
|     --> /pdfium/src/bindgen/pdfium_6666.rs:1898:33
|      |
| 1898 |     ["Size of _UNSUPPORT_INFO"][::std::mem::size_of::<_UNSUPPORT_INFO>() - 16usize];
|      |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempt to compute `8_usize - 16_usize`, which would overflow
| 
| error[E0080]: evaluation of constant value failed
|     --> /pdfium/src/bindgen/pdfium_6666.rs:1939:30
|      |
| 1939 |     ["Size of _IFSDK_PAUSE"][::std::mem::size_of::<_IFSDK_PAUSE>() - 24usize];
|      |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempt to compute `12_usize - 24_usize`, which would overflow
| 
| error[E0080]: evaluation of constant value failed
|     --> /pdfium/src/bindgen/pdfium_6666.rs:2514:34
|      |
| 2514 |     ["Size of _IPDF_JsPlatform"][::std::mem::size_of::<_IPDF_JsPlatform>() - 104usize];
|      |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempt to compute `52_usize - 104_usize`, which would overflow
| 
| error[E0080]: evaluation of constant value failed
|     --> /pdfium/src/bindgen/pdfium_6666.rs:2850:36
|      |
| 2850 |     ["Size of _FPDF_FORMFILLINFO"][::std::mem::size_of::<_FPDF_FORMFILLINFO>() - 280usize];
|      |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempt to compute `140_usize - 280_usize`, which would overflow
| 
| error[E0080]: evaluation of constant value failed
|     --> /pdfium/src/bindgen/pdfium_6666.rs:3675:33
|      |
| 3675 |     ["Size of FPDF_FILEWRITE_"][::std::mem::size_of::<FPDF_FILEWRITE_>() - 16usize];
|      |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempt to compute `8_usize - 16_usize`, which would overflow
| 
| error[E0080]: evaluation of constant value failed
|     --> /pdfium/src/bindgen/pdfium_6666.rs:3853:31
|      |
| 3853 |     ["Size of _FX_FILEAVAIL"][::std::mem::size_of::<_FX_FILEAVAIL>() - 16usize];
|      |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempt to compute `8_usize - 16_usize`, which would overflow
| 
| error[E0080]: evaluation of constant value failed
|     --> /pdfium/src/bindgen/pdfium_6666.rs:3886:35
|      |
| 3886 |     ["Size of _FX_DOWNLOADHINTS"][::std::mem::size_of::<_FX_DOWNLOADHINTS>() - 16usize];
|      |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempt to compute `8_usize - 16_usize`, which would overflow
| 
| error[E0080]: evaluation of constant value failed
|     --> /pdfium/src/bindgen/pdfium_6666.rs:4315:35
|      |
| 4315 |     ["Size of _FPDF_SYSFONTINFO"][::std::mem::size_of::<_FPDF_SYSFONTINFO>() - 72usize];
|      |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempt to compute `36_usize - 72_usize`, which would overflow
| 
| error[E0080]: evaluation of constant value failed
|     --> /pdfium/src/bindgen/pdfium_6666.rs:4349:38
|      |
| 4349 |     ["Size of FPDF_CharsetFontMap_"][::std::mem::size_of::<FPDF_CharsetFontMap_>() - 16usize];
|      |                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempt to compute `8_usize - 16_usize`, which would overflow

[ajrcarey]

Trial and error showed that a change in bindgen introduced in 0.70.0 causes the build failures. Locked bindgen version in Cargo.toml to 0.69.4 while investigation continues. It may be necessary to file an issue upstream at https://github.com/rust-lang/rust-bindgen.

[ajrcarey]

Added notes to Cargo.toml and README. Change will be included in release 0.8.25.