go/types: make typeset return an iterator

go/types: make typeset return an iterator

typeset(t) now returns a func equivalent to iter.Seq2[Type, Type]
for the sequence over (type, underlying) pairs in the typeset of t.

underIs was modified to take advantage of the underlying iteration
primitive, all, which computes the desired boolean conjunction
directly.

Change-Id: I3e17d5970fd2908c5dca0754db3e251bf1200af2
Reviewed-on: https://go-review.googlesource.com/c/go/+/688876
Auto-Submit: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>