cmd/compile/internal/types2: implement Checker.implements

cmd/compile/internal/types2: implement Checker.implements

Checker.implements implements the complete interface
"implements" predicate. Use it instead of Checker.satisfies.
This is mostly a refactoring of the code but the constraint
types have already been instatiated with the respective type
arguments as needed before calling "implements".
Future CLs will address the various TODOs.

Change-Id: If530cca36643a561282361348f1526157a7182de
Reviewed-on: https://go-review.googlesource.com/c/go/+/363837
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>