Function references unresolved issues

[dhil]

PR #5288 implements the function references proposal modulo a few missing bits. The missing bits are:

• [x] Support for function references in the embedder API.
• [x] Failing tests:
  • [x] type-equivalence.wast: type canonicalisation is currently broken due to #[derive(hash)] not considering that two distinct typed function reference indices may point to structurally equivalent types.
  • [x] br_table.wast: this fails due to incomplete support for the various syntactic sugar in the wasm-tools/wast crate. See bytecodealliance/wasm-tools/pull/952.
  • [x] ref_null.wast and local_init.wast: fail due to lack of support in the embedder API.
  • [x] return_call_ref.wast, return_call_indirect.wast, and return_call.wast: fail due to lack of tail call support.

At the time of writing the implementation does not take advantage of the static non-null safety to elide null checks when executing call_ref: https://github.com/bytecodealliance/wasmtime/blob/36e9f7e577df11de14f5973308c4b4bd0565eed9/crates/cranelift/src/func_environ.rs#L1744-L1762

[fitzgen]

I've finished support for function references over in https://github.com/bytecodealliance/wasmtime/pull/7943