go/types: import instance.expand and subst.go from dev.go2go

Changes from dev.go2go:
 + A potentially latent bug is fixed when nilling out tparams in an
   instantiated signature (the resulting type could be Typ[Invalid])
 + Support for pointer designation is removed
 + instantiatedHash is updated to use '[]' rather than '()'
 + Several TODOs were added for me to follow-up on, rather than address
   in this CL
 + Error callsites are updated. Deciding on error codes and better error
   messages is punted to a later CL

These changes can be reviewed by comparing with Patchset #1 of this CL.

Change-Id: Ib5869586b8395419013010e2085cab877727d2ef
Reviewed-on: https://go-review.googlesource.com/c/go/+/276253


Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>