cmd/compile: add support for generic maps

Add support for maps in subster.typ(). Add new test cases maps.go and set.go.

Change substitution of a TFUNC in subster.typ() to always create new
param and result structs if any of the receiver, param, or result
structs get substituted. All these func structs must be copied, because
they have offset fields that are dependent, and so must have an
independent copy for each new signature (else there will be an error
later when frame offsets are calculated).

Change-Id: I576942a62f06b46b6f005abc98f65533008de8dc
Reviewed-on: https://go-review.googlesource.com/c/go/+/301670


Trust: Dan Scales <danscales@google.com>
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>