Use constants instead of dynamically computed values to determine
the bit sizes of types similar to how strconv and other packages
directly compute these sizes. Move these constants near the code
that uses them.

Change-Id: I78d113b7e697466097e32653975df5990380c2c1
Reviewed-on: https://go-review.googlesource.com/20514
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>