Add ECMH multiset module to libsecp256k1

Summary:
This can be used for UTXO commitments.

Specification at
https://github.com/tomasvdw/bips/blob/master/ecmh.mediawiki

Test Plan: Tests included

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Subscribers: schancel, matiu, deadalnix, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1072