[dev.typeparams] go/types: move signature checking into separate file
This is a port of CL 321590 to go/types. Specifically, the same checker
methods were moved.
Change-Id: If4522d316f29c6b6f887580aa037e6b6dedbb6ac
Reviewed-on: https://go-review.googlesource.com/c/go/+/324754
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Robert Griesemer <gri@golang.org>