go/types: implement deduplication of instances using the Environment

Implement deduplication of type instances via the Environment by
recording a map of unique IDs for *Named types. This avoids an issue
with the existing type hash, where qualified names alone were not
sufficient to differentiate two *Named types that have the same fully
qualified name but which are distinct pointers. It also allows us to
drop the scope accounting for local types.

A subtle bug is also fixed in subst.go, where the instance t was passed
to typeHash rather than t.orig.

Change-Id: I85639ccc1c9bfee470babd2fc85375484c8ed0b9
Reviewed-on: https://go-review.googlesource.com/c/go/+/344390


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>