To support transient signing pubkeys and payer ids for Offers, add
another key derivation to ExpandedKey. Also useful for constructing
metadata for stateless message authentication.
Adds two new payment `Method`s for identifying payments with custom
`min_final_cltv_expiry_delta` as payments with LDK or user payment
hashes.
The `min_final_cltv_expiry_delta` value is packed into the first 2
bytes of the expiry timestamp in the payment secret metadata.
As part of preparing to expose some of its methods as pub for ChannelManager-less
phantom invoice generation.
Pure code move of the module + the addition of module-level documentation
