This introduces the signed-digit multi-comb multiplication algorithm
for constant-time G multiplications (ecmult_gen). It is based on
section 3.3 of "Fast and compact elliptic-curve cryptography" by
Mike Hamburg (see https://eprint.iacr.org/2012/309).
Original implementation by Peter Dettman, with changes by Pieter Wuille
to use scalars for recoding, and additional comments.
