cmd/compile: implement range over func

cmd/compile: implement range over func

Add compiler support for range over functions.
See the large comment at the top of
cmd/compile/internal/rangefunc/rewrite.go for details.

This is only reachable if GOEXPERIMENT=range is set,
because otherwise type checking will fail.

For proposal #61405 (but behind a GOEXPERIMENT).
For #61717.

Change-Id: I05717f94e63089c503acc49b28b47edeb4e011b4
Reviewed-on: https://go-review.googlesource.com/c/go/+/510541
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Auto-Submit: Russ Cox <rsc@golang.org>