Backport secp256k1 #428

Summary:
The is backport from core.

Also amen .arclint so that we don't format 3rd party code.

Test Plan: Run the test suite.

Reviewers: freetrader, sickpig, MattBitprim

Reviewed By: MattBitprim

Differential Revision: https://reviews.bitcoinabc.org/D117