[dev.typeparams] cmd/compile/internal/types2: move methods on *Named into named.go (cleanup)

No other code changes except for an additional comment.

Change-Id: Ica3cea446c6c88f4f81a86d77b289a0b54b1e76f
Reviewed-on: https://go-review.googlesource.com/c/go/+/333671


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>