Make Web Assembly a Tier 1 target

[teryror]

I have worked with WASM barely at all, but from what I know about it, it seems like an environment coca is well suited to. Since it's the way for Rust to target the browser, it's probably worth giving some careful attention.

• Ensure compatibility going forwards. As of the time of writing, coca at least builds with --target wasm32-unknown-unknown. This should be checked in CI, and we'll also want to run the test suite using wasm-bindgen-test.
• Make using coca in this environment as painless as possible. Some investigation is required on what exactly this entails - some WASM-specific utilities, most likely, such as a function for conveniently setting up an Arena, for example.
• Documentation: Make sure that this is clearly communicated in the README (it's probably be a major selling point to some), and include a WASM-specific "Getting Started" section in the docs.