[dev.typeparams] go/types: implement type terms

This is a port of CL 338049 to go/types. It identical to that CL, except
for eliding unnecessary typenames from the testTerms declaration.

Change-Id: Ieb04d7bbc20063044eb63ea985f75d529f030cd7
Reviewed-on: https://go-review.googlesource.com/c/go/+/339653


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>