go/types: use []*TypeParam rather than []*TypeName type param lists

go/types: use []*TypeParam rather than []*TypeName type param lists

Making this change improves type safety slightly, and avoids many
internal type assertions.

Change-Id: I26519b0e57068e944e8243983ae90553d79e59c2
Reviewed-on: https://go-review.googlesource.com/c/go/+/343932
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>