[dev.typeparams] cmd/compile/internal/types2: remove Type.Under method in favor of function

This removes the need for the aType embedded type and brings the types2.Type
API in sync with the go/types.Type API.

For reasons not fully understood yet, introducing the new under function
causes a very long initialization cycle error, which doesn't exist in
go/types. For now, circumvent the problem through a helper function variable.

This CL also eliminates superflous (former) Under() method calls
inside optype calls (optype takes care of this).

Plus some minor misc. cleanups and comment adjustments.

Change-Id: I86e13ccf6f0b34d7496240ace61a1c84856b6033
Reviewed-on: https://go-review.googlesource.com/c/go/+/293470


Reviewed-by: Robert Findley <rfindley@google.com>
Trust: Robert Griesemer <gri@golang.org>