✨ Merged: NUT-14: Hashed Timelock Contracts (HTLCs)

HTLCs allow you to attack spending conditions to Cashu Ecash the similar to how it works with Lightning. There are two ways to spend the ecash: either you know the preimage to a hash, or you wait until the timelock expires. Both conditions can be combined with an additional public key that needs to sign the token for spending it (same as with P2PK tokens).

This allows us to build awesome things like:

- Atomic swaps between ecash (of different mints for example)

- Combine it with Lightning HTLCs to extend payment routes over an Ecash mint

- Build trustless Lightning gateways that can serve as Lightning payment providers for users of a mint

- Probably more cool stuff that I can't think of