go/types: implement Checker.implements

This is a port of CL 363837 from types2 to go/types. As usual, test
error messages had to be repositioned on the operand.

Change-Id: I2b53fae7aa30f9147f8d05f75b0ab252338320bb
Reviewed-on: https://go-review.googlesource.com/c/go/+/364934


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>