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It was too complicated, assuming the syntax is more general than reality.
It must be a possibly negative integer followed by an optional minus sign
and positive integer. Literals only, no expresssions.

Also put in a TODO about address parsing and clean up a couple of types.

Change-Id: If8652249c742e42771ccf2e3024f77307b2e5d9a
Reviewed-on: https://go-review.googlesource.com/3370

Reviewed-by: Russ Cox <rsc@golang.org>