The current modInverse implementation allocates a big.Int
for the second parameter of GCD, while only the first is needed.
This is unnecessary and can lead to a speed up for optimizations
of GCD where the second parameter is not calculated at all.

Change-Id: I3f042e140ff643311bc3d0b8d192992d4d2c4c70
Reviewed-on: https://go-review.googlesource.com/50531
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filosottile.wiki@gmail.com>
Reviewed-by: Adam Langley <agl@golang.org>