go/types: implement generic conversions

This is a port of 4 CLs from types2: CL 356010, CL 357333, CL 357410,
and CL 357249. These 4 CLs are all related to implementing conversions,
and porting them together saved time (particularly because go/types was
already threading a *reason argument in some places).

Change-Id: Ic89b608d7096b61bfb9f7d71fdae2cc50b0ed70e
Reviewed-on: https://go-review.googlesource.com/c/go/+/359137


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