rdar://101241285

When typed function references are enabled, the TypeKind of the Funcref and Externref types becomes RefNull. This causes examples such as

    (module
      (type (;0;) (func (result i32)))
      (func (;0;) (type 0) (result i32)
        (local externref funcref)
        local.get 0
        i32.const 1
        table.grow 0)
      (table (;0;) 500 2000 funcref)
      (table (;1;) 500 2000 externref))

to result in unhelpful error messages like `table.grow expects fill value of type RefNull got RefNull, in function at index 0`. 

Since typed function references are just one instance of the WebAssembly type system becoming more complex in newer proposals, this is probably a good opportunity to improve our error messages in general to log more of the available type information.