Merge #435: Add functional style methods to various keys

12d4583 Implement negate that consumes self (Tobin Harding)
5eb2d74 Rename tweak_add_assign -> add_tweak (Tobin Harding)
b9d08db Replace _assign with _tweak (Tobin Harding)

Pull request description:

  The various `_assign` methods (`add_assign`, `add_expr_assign`, `mul_assign`, `tweak_add_assign`) are cumbersome to use because a local variable that uses these methods changes meaning but keeps the same identifier. It would be more useful if we had methods that consumed `self` and returned the newly modified type.

  We notice also that this API is for adding/multiplying tweaks not arbitraryly adding keys.

  - Patch 1: Changes add/mul_assign -> add/mul_tweak for `PublicKey` and `SecretKey` (incl. re-working unit tests)
  - Patch 2: Changes `tweak_add_assign` -> `add_tweak` for `KeyPair` and `XOnlyPublicKey`
  - Patch 3: Changes `negate_assign` -> `negate`

  All methods changed include:
  - New method consumes self and returns the tweaked key
  - Original  method remains with a `deprecated` attribute, however I've left a TODO in there for adding the `since` field.

  Close: #415

ACKs for top commit:
  apoelstra:
    ACK 12d4583

Tree-SHA512: 026e8722892f3a0f18956281e4d2356d2789ef535a7ab71a375758201b180663d068397cde2dca5f60858ab7158069e53d7096326bfbd5a364269b0be680940c

Author: apoelstra