lz4-sys: add libc shim for wasm targets

[james-rms]

This crate does not build for WASM targets currently. This patch is intended to get builds for wasm32-unknown-unknown working. It follows a similar approach to that used in https://github.com/gyscos/zstd-rs .

• Adds C header files that are included from the parts of liblz4 that lz4-sys depends on. This includes stdlib.h, string.h and assert.h. The new experimental lz4file component of liblz4 depends also on stdio.h, but this library does not wrap that component.
• Adds basic rust implementations only for the functions used within liblz4. These are malloc, free, memcmp, memmove, memcpy. One file also uses the assert macro- this patch defines the macro as a no-op.
• exposes the C primitive types used in the lz4-sys API publically from that crate, and uses them from lz4-rs instead of depending on libc separately. This shouldn't have a real effect on what dependencies are built when, but helps consolidate the platform-specific logic only to lz4-sys.
• Adds a check in CI to build and check for wasm32.

[pmarks]

Very cool, thanks! Is there an easy way to also run the unit tests in CI?

[pmarks]

Also the wasm build doesn't seem to work on MacOS.

[james-rms]

@pmarks I have it working on my local mac, but the Apple-vendored clang binary doesn't build wasm32 out of the box, you need to install llvm separately. Will try to get this working on the CI runner.

Running tests is a good idea, i'll try to figure out how.

[james-rms]

@pmarks the steps to get tests working in WASM are (roughly following this guide):

• install wasm-pack and node on the CI runner
• write new tests specifically for wasm and wrap them with #[wasm-bindgen-test]. These maybe don't need to be exhaustive, just enough to exercise the APIs.
• run them in CI with wasm-pack test

IMO this is overkill, but i'm willing to give it a go if you think it's neccessary.

[pmarks]

I'm ok merging without having the unit tests in CI. Seems a bit risky that we could somehow break WASM if we're not running the tests since it's the only 32bit target so you can have bugs from e.g. assuming usize is 64 bits.

Doesn't seem too bad to run the tests? Node should already be installed - maybe we can do it in a follow up PR.

Also I'm OK disabling the WASM build on Mac if it's a pain to get working.

[james-rms]

@pmarks i think it's not a pain, but it's hard to iterate on when every CI run requires approval. I'll see if I can get CI running in my fork first.

[james-rms]

@pmarks OK, macOS CI should be green.

[pmarks]

@james-rms Thanks!!

[james-rms]

@pmarks Thanks for the review! I'd appreciate it if you drop an @ here when you get around to cutting a release with this commit.