With this commit we add a new way to encrypt and decrypt a sensitive
payload: By using Diffie-Hellman over a local and remote key to derive
at a shared secret key.
The functions inside of the crypto.go file in chanbackup (like EncryptPayloadToWriter and DecryptPayloadFromReader) can be used by a lot of things outside of just the chanbackup package. We can't just reference them directly from the chanbackup package because it's likely that it would generate circular dependencies. Therefore we need to move these functions into their own package to be referenced by chanbackup and whatever new functionality that needs them
