libweave: Imported p225_spake implementation from Chrome

Exact copy from
https://crrev.com/3e4cf54491803eb102c4887f1de5ccd088c612b1

BUG=brillo:1270
TEST=none, nothing is part of the build yet.

Change-Id: I048b0763c32ef58c459b69a9f9f94526ec3dd038
Reviewed-on: https://chromium-review.googlesource.com/291428
Reviewed-by: Vitaly Buka <vitalybuka@chromium.org>
Tested-by: Vitaly Buka <vitalybuka@chromium.org>