Add C FFI Bindings

[mcb2003]

This PR adds extern "C" bindings to all functionality of the TTS struct, plus that of Features and Backends. The bindings are gated behind a new "ffi" Cargo feature.

Error Handling

Since we don't have the luxury of Result<T, E> in C, I created a thread-local static to store the last error in string form, and two functions:

• tts_get_error() to retrieve any errors.
• tts_clear_error() to deallocate the last error (if any). This is probably unlikely to get used that often, since Rust's CString will deallocate itself when dropped.

The caller can then determine if an error has occurred by examining the return value of most functions (if it is NULL, or false, something went wrong).

Generating the Bindings

I used cbindgen to create the C headers in testing, so included is a cbindgen.toml with some relatively sane settings.

One problem is detecting when we're compiling for iOS. I haven't found any way to do this that can be explained to cbindgen, so the headers would have to be modified manually. Consequently, the Apple section of the Backends enum currently looks like this:

#if defined(__APPLE__)
  BACKENDS_APP_KIT,
#endif
#if defined(__APPLE__)
  BACKENDS_AV_FOUNDATION,
#endif

Limitations

• Allowing Rust to call C callback functions to notify on utterance start, end and stop seems pretty complex, namely because the on_utterance_* functions take a Boxed FnMut wide pointer.
• When the static last_error CString is updated, a CString is constructed from a Rust String. Since there's no good way to losslessly get rid of '\0' bytes in the middle of a Rust String when converting to CString that I know of, if any error messages contain null bytes, Rust will panick and the result will be UB.

[mcb2003]

I've now modified build.rs to automatically generate the C bindings and save them to $OUT_DIR/tts.h.

I've also fixed the failing wasm test. This was caused by libc not defining c_float and c_char for wasm32-unknown-unknown targets because, well, there is no libc for that target. See commit b972f44 and this libc crate issue for more info.

[Keithcat1]

Can this be murged? It would be useful to use this in other languages because it not only provides cross-platform speech, but handles WinRT on Windows in a convenient manner, which can't be done in less mature languages like Dart or Go.

[ndarilek]

Haven't looked into it, but was a port of at least the basic hello_world example ever added? At the very least I need something I can build as part of CI to ensure that the API remains compatible. Without that in place, I have no way of knowing whether future changes break the API since I myself am not using the C/C++ interface.