Add pippenger_wnaf for multi-multiplication

Summary: Backport PR486 and PR510

Test Plan:
Run bench_ecmult after building both with autotools and cmake.
Run secp256k1 test suite.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D4833