go/types: eliminate typeHashing global variable

go/types: eliminate typeHashing global variable

This is a port of CL 345929 to go/types. It is also a step toward making
instantiation concurrency-safe.

Also fix some whitespace in instantiate.go.

Updates #47910

Change-Id: Icdeb227cb83eee15da6db90daab294c8c55db601
Reviewed-on: https://go-review.googlesource.com/c/go/+/346557
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>