Mostly suggested by Alan.
Convert Const* ops to just one Const op.
Use more of go/types.
Get rid of typers, all types must be specified explicitly.

Change-Id: Id4758f2b887d8a6888e88a7e047d97af55e34b62
Reviewed-on: https://go-review.googlesource.com/8110Reviewed-by: Alan Donovan <adonovan@google.com>