Feature request: support for wasm externref and funcref types

[TheZipCreator]

WebAssembly has the externref and funcref types, the former opaquely represents an external object (usually from javascript), and the latter represents an external function. Currently, there's no way to access these in D when compiling to a wasm target. It'd be nice to have an externref_t or something to use them, kinda similar to what I think clang does.

It'd be nice to also have support for wasm tables, but this isn't a necessity.