This eliminates all direct reads of Type.Bound
outside type.go.

Change-Id: I0a9a72539f8f4c0de7f5e05e1821936bf7db5eb7
Reviewed-on: https://go-review.googlesource.com/21421
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>