go/types: implement singleType and structure (type)

go/types: implement singleType and structure (type)

This is a clean port of CL 359015 to go/types.

Change-Id: Iea4e3bfe0a4ae0e5a9052cb6e66c01405bd57c3d
Reviewed-on: https://go-review.googlesource.com/c/go/+/360756
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>