This commit adds new `wasmtime_*` symbols to the C API, many of which
mirror the existing counterparts in the `wasm.h` header. These APIs are
enhanced in a number of respects:

* Detailed error information is now available through a
  `wasmtime_error_t`. Currently this only exposes one function which is
  to extract a string version of the error.

* There is a distinction now between traps and errors during
  instantiation and function calling. Traps only happen if wasm traps,
  and errors can happen for things like runtime type errors when
  interacting with the API.

* APIs have improved safety with respect to embedders where the lengths
  of arrays are now taken as explicit parameters rather than assumed
  from other parameters.