POC print variant runtime repr in error messages

[cristianoc]

Test:

module M: {
  @unboxed type t = | @as(null) A
} = {
  @unboxed type t = | @as(undefined) A
}

Triggered in error message:

Type declarations do not match:
    type t = @as(undefined) A
  is not included in
    type t = @as(null) A

Still does not print @unboxed.