Merge #173: musig-spec: Add motivation and design sections

802b7da musig-spec: add motivation and design sections (Jonas Nick)
686d962 musig-spec: various cleanups (Jonas Nick)
ef537b2 musig-spec: fix unnecessary O(n^2) KeyAgg runtime (Jonas Nick)

Pull request description:

ACKs for top commit:
  real-or-random:
    ACK 802b7da

Tree-SHA512: 03e69f8a4028411454f9e3533989de170abd788a964fd12d7b15f75768aecbf34e64f12d02c673732279a9748844481185721b5c02fd7cca8ee6d7c37e3c194c

Author: jonasnick