[dev.typeparams] go/types: move Signature type decl into signature.go (cleanup)

This is a port of CL 332091 to go/types. Notably types.Signature is
missing APIs for RParams. This is left for a later CL, when the API is
finalized.

Change-Id: I70b664d656e8c1e32958a75e1d13eab5f3281bf9
Reviewed-on: https://go-review.googlesource.com/c/go/+/335038


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>