Sync code with Google-internal version.

This brings the constrainedcrypto code back in sync with the
respective Google-internal copy of this code. Changes are mostly
cosmetic (some explicit casts, some int -> unsigned int changes), the
only major change is additional p256 code. We don't use the p256 code
right now, but I'm adding it in an effort to keep the files from
diverging too much.

BUG: 27287913
Change-Id: Ie8672bbb2487d576ab4c5f6dda4ab6c194575985
8 files changed