WIT: consider dropping `union`s

[lann]

Unions are uniquely challenging to implement in bindings because their cases do not have kebab-case names. See for example the wit-bindgen Rust generator's awkward solution to this: https://github.com/bytecodealliance/wasmtime/blob/367bdc8c66df62d6854966a90564b42692a3f023/crates/wit-bindgen/src/rust.rs#L317

Rather than expecting each bindings language to come up with a generic naming scheme to handle unions, I think we should put the onus on WIT authors to select reasonable case names on an equivalent variant.

A quick Github search (and review of WASI) suggests that unions are not currently in use outside of tests and samples.

[alexcrichton]

Personally I'd agree with this and would be in favor 👍

[lukewagner]

I'd be fine dropping them too. Initially they were added because the proposal spawned out of Web IDL, which has union. But yeah, I can see how it ends up being a special case for many languages, especially the statically-typed ones.

[guybedford]

Even in JavaScript, they're still tagged awkwardly and don't actually correspond to the IDL / TypeScript equivalents, so this seems fine to me as well.