cmd/compile/internal/types: unexport New and NewBasic

Now that the universe is fully initialized within package types, we
can stop exporting New and NewBasic, which are only needed for that
purpose. So this CL renames "New" to "newType" and "NewBasic" to
"newBasic".

This CL also moves the initialization of Types[TBLANK] and Types[TNIL]
from typecheck.InitUniverse to types.InitTypes, which I missed in an
earlier CL. And a use of "New(TSTRING)" in test/abiutils_test.go,
which should just be "Types[TSTRING]" anyway.

Change-Id: I1d83f93e27b88be289d4f3f6c16357a20f570460
Reviewed-on: https://go-review.googlesource.com/c/go/+/345487


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