Add trivial ecmult_multi algorithm which does not require a scratch space

Summary:

• 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

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4985