feat: Support compilation as wasm32-unknown-unknown

[dhdaines]

Currently in order to get rustfst to run in the browser it's necessary to build it as WASI and then stub out the imported WASI symbols (using https://github.com/typst-community/wasm-minimal-protocol/tree/main/crates/wasi-stub for instance)

This is suboptimal since it adds an extra build step and retains useless code in the binaries. But unfortunately it's not actually possible to build rustfst-ffi for the bare metal WebAssembly target wasm32-unknown-unknown for a couple of reasons:

1. randgen requires getrandom which doesn't work on wasm32-unknown (there is a way around this but it's discouraged for libraries: https://docs.rs/getrandom/latest/getrandom/#webassembly-support)
2. ffi-convert doesn't compile for wasm32-unknown-unknown because of a (useless) dependency on libc (see fix: remove dep on libc to make wasm unknown work sonos/ffi-convert-rs#68), and libc is also (uselessly) used in a lot of places in rustfst-ffi
3. obviously there are a few file I/O operations for loading and saving symbol tables and FSTs and such!

(I realize using libc is probably leftover from when std::ffi and core::ffi weren't very good, as the now very dated "Rust FFI Omnibus" suggests to do)

This PR adds a std default feature which compiles these things that require stdio. So to build for wasm32-unknown-unknown you can omit the default features, the same pattern as we see elsewhere:

cargo build --package rustfst-ffi --no-default-features --target wasm32-unknown-unknown

(--features rustfst-state-label-u32 is unnecessary since wasm32 is 32)
(doesn't work for the top-level build because Reasons)

Because there isn't yet a version of ffi-convert that compiles, and when it will be released it will require Rust 2024, and I'm not sure if you want to require that here, I've simply vendored a fixed version of it since the license is totally compatible.

Note: changing libc::size_t to usize has revealed that ... why exactly are you using usize for all those things in rustfst-ffi? Possibly for some of them it should be the same type as state IDs which I think is either usize or u32 (of course these are the same thing on WebAssembly!)

[dhdaines]

Changed to draft! No longer necessary to vendor ffi-convert-rs as it has been updated, so I will update this as soon as possible: sonos/ffi-convert-rs#68 (comment)