cmd/compile/internal/types2: export TypeHash, return value without blanks

Change the typeWriter to produce blank-free hashes where easily possible
if used as a type hasher, and replace remaining blanks with '#' is needed.

Exported Environment.TypeHash for use by the compiler.

Change-Id: Icbd364c207f9c139a7a1844bb695512a0c56a4e4
Reviewed-on: https://go-review.googlesource.com/c/go/+/349990


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