go/types: use a typeWriter to write types (cleanup)

go/types: use a typeWriter to write types (cleanup)

This is a port of CL 345890 to go/types.

Change-Id: I98162deaf044b2194b05dc51e6948e227216fc4d
Reviewed-on: https://go-review.googlesource.com/c/go/+/346554
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>