[dev.typeparams] go/types: introduce type set abstraction for interfaces
This is a port of CL 329309 to go/types, with minor updates for API
differences and to handle methodset.go, which doesn't exist in types2.
A couple pre-existing comments were adjusted to match types2.
Change-Id: I3fd556e1326013a694ff5edb8518ca24c27bd10b
Reviewed-on: https://go-review.googlesource.com/c/go/+/334894
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>