Update multiset hash benchmark to use get_iters

Summary:
Also take the opportunity to add it to travis and make the style a bit more consistent.

Depends on D6364

Test Plan:

ninja bench-secp256k1-multiset

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6372