Add type validations for function imports

[stevenfontanella]

Validates that the exported function is a subtype of the import declaration. Part of #8261.

Example error:

(module $A
  (func $f (export "f") (param i32) (result i32)
    (local.get 0)
  )
)

(register "A" $A)

(module $B
  (func (import "A" "f") (result i32))
)

;; [trap Imported function A.f with type (func (param i32) (result i32)) isn't compatible with import declaration with type (modulo rec groups): (func (result i32))]

[tlively]

As a follow-up, it would be nice to give Importable a getKind method so we could use a switch here.

[stevenfontanella]

We could use std::visit or switch on import.index() if that sounds good? I didn't use visit originally since there are a lot of variables that we'd have to capture from the outside. Is the idea to make the cases exhaustive?

[tlively]

Yes, to make it exhaustive and to make it easier to read (although that's subjective). I guess we would still need to cast to the relevant subtype, though, so maybe it's not worth it.