reference types for typed functions

[WhaleKit]

Currently neither Extern::typed_func nor ModuleInstance::exported_func can have references from "reference types" proposal in their signatures. In this PR i'm adding 2 newtypes for that usage, with from/into underlying u32, but not deref.

I briefly considered making FuncAddr and ExternAddr themselves into newtypes (from aliases), but that would break existing public api.

Also includes example with function reference being passed around as a callback

[explodingcamera]

Thanks for the PR! I'll have to think about this a bit more and check how other engines handle this. I think replacing the FuncAddr and ExternAddr in WasmValue with a newtype (e.g FuncRef and ExternRef) is probably the way to go, as the address should be opaque (the user probably shouldn't be able to pass in their own u32), and the next version will have breaking changes anyways.

edit: I changed some things around with WasmValue, I'll come back to this pr to incorporate your example soon, thank you!

[WhaleKit]

Updated. This PR now only has example of using function references in typed (exported and imported) functions, which I updated to reflect the new types added in next.