c1eccfde25 Move Witness to primitives (Tobin C. Harding)
6ce76cd7c8 Add unstable constructor to Witness (Tobin C. Harding)
Pull request description:
Patch 1 introduces a new policy to the codebase, we use `foo__unstable` for public unstable functions and there are zero semver guarantees if you call these functions.
Patch 2 does the move.
Close#3406
ACKs for top commit:
apoelstra:
ACK c1eccfde25fd4c2b19e7ec6759352b46ac246113; successfully ran local tests
Tree-SHA512: 2388066be2b6bb2cf3d6757c8f6334beeda6115ef1ce7c537955d32aa5e466add5162d0d2adee27f416fe622fc93c4e94bd848326463ee55e08d1c0f4e03719c