[dev.typeparams] go/types: import the Type API from dev.go2go

Import the Type changes from the dev.go2go branch, as well as a minimal
set of related changes to the Checker code so that tests pass. This
involved making some decisions about which functionality to import, with
some parts of the Type API (for example instance.expand) carved out as
they pulled in too many additional changes.

Minimal changes were made from the dev.go2go version. Notably:
 + The aType helper struct is removed. It could have been removed in CL
   274852 but was missed.
 + writeTParamList was cleaned up for the bracketed parameter style.
 + Some interface iteration functions were simplified.

One observation along the way is that, especially with converter methods
removed from the Type interface, we should probably avoid storing the
Checker on Type instances for lazy evaluation. If a function is only
valid during a checking pass, we should make it a method on Checker
instead of a method on the Type instance. Rather than do this now, which
would make later changes harder to import, I left a couple TODOs to
address this later.

Change-Id: Ie669224614269874474d87e46c68216cb06b6d0e
Reviewed-on: https://go-review.googlesource.com/c/go/+/275441


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