go/types: generalize assignability to generic types

This is a port of CL 357917 to go/types. Some error messages in
assignability.go2 had to be adjusted. I left a TODO to investigate
whether we should align error messages.

Change-Id: Ia323ffe18bc08e82de62044f35b8b0f3edd7dc08
Reviewed-on: https://go-review.googlesource.com/c/go/+/360476


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>