[dev.typeparams] go/types: move (remaining) type decls into their own files (cleanup)

[dev.typeparams] go/types: move (remaining) type decls into their own files (cleanup)

This is a port of CL 332093 to go/types. A missing comment is added to
named.go, and some TODOs were added to converge on the TypeParam API.

Change-Id: I781a1d0d3fc6c11bb323123e954c106094d998ef
Reviewed-on: https://go-review.googlesource.com/c/go/+/335040
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>