bip32: add PrivateKey::derive_tweak() and PublicKey::derive_tweak()

[fjarri]

Closes #1185

Adds PublicKey::derive_tweak() and PrivateKey::derive_tweak() exposing the tweak-generation API that extended keys use. These methods have default implementations, so it's not a breaking change. This is the minimum that I need to use BIP-32 with DKG shares.

Alternatively, these can be separate functions, to illustrate that the implementation is standardized and should not be overridden.

Possible further work:

• Add methods to generate a tweak chain for the given derivation path and apply it to a PrivateKey or PublicKey. I currently do it manually in my own code.
• Wrap the tweak in its own ZeroizeOnDrop-able type instead of using plain PrivateKeyBytes. This will be a breaking change.
• Implement the logic generically using ecdsa and elliptic-curve traits. This will be a major breaking change, given how many hardcoded constant-sized arrays are there now. In particular this will allow us to have tweaks of Scalar type instead of just byte arrays, which will mean more localized error checking and an ability to sum them.

[tony-iqlusion]

This seems fine as a general direction. I'll try to do a more detailed review of it soon.

[fjarri]

Thanks! Would you mind cutting a release too?