[dev.typeparams] cmd/compile: clean up instantiation and dictionary naming

Separate generation of instantiation and dictionary name generation.

Add code to add subdictionaries to a dictionary. Not quite working
yet, as we need to trigger generation of the subdictionaries for methods.

Change-Id: I0d46053eba695b217630b06ef2f990f6a0b52d83
Reviewed-on: https://go-review.googlesource.com/c/go/+/331209


Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>