Get rid of (*Mpint).Add's "quiet" parameter: it's always set to 0.

Inline (*Mpint).shift into (*Mpint).Lsh and (*Mpint).Rsh. There's no
need for a common shift method that can handle both left or right
shifts based on sign when the higher level abstractions only ever do
one or the other.

Change-Id: Icd3b082413f9193961b6835279e0bd4b6a6a6621
Reviewed-on: https://go-review.googlesource.com/21050
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>