Merge #167: Add ordinary and x-only tweaking to spec and simplify imp…

…lementation

eac0df1 musig: mention how keyagg_cache tweak and parity relate to spec (Jonas Nick)
57eb6b4 musig-spec: move description of secret key negation to spec (Jonas Nick)
633d01a musig-spec: add x-only and ordinary tweaking to musig (Jonas Nick)
aee0747 musig-spec: add general description of tweaking (Jonas Nick)
fb060a0 musig-spec: add Session Context to simplify sign/verify/sigagg (Jonas Nick)
3aec433 musig-spec: move remarks on spec below specification section (Jonas Nick)
628d52c musig-spec: fix title/abstract and make algo names bold (Jonas Nick)
5b760cc musig-spec: consistently call partial sigs psig (Jonas Nick)
f0edc90 musig: fix number of tweaks in tweak_test (Jonas Nick)

Pull request description:

ACKs for top commit:
  real-or-random:
    ACK eac0df1 -- I haven't checked all the indices etc, so this is more of a Concept ACK than a "pseudocode review ACK" but we we have the ACK by Brandon and this is anyway still a draft, so I think this is good to be merged.

Tree-SHA512: 9e16e7892e103205d96060158a7a6c01480d2b59300bbf9f0655b4d26586e632be8b8f656fe07c7ece1421ec91e0b387d6fcf363db7aedc0402d265b1d9df474