Add trivial ecmult_multi algorithm which does not require a scratch space
- Add trivial ecmult_multi algorithm. It is selected when no scratch space is given and just multiplies and adds the points.
- Add trivial ecmult_multi to the benchmark tool
This is a backport of secp256k1's PR580
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D4985