Lightning invoices allow for zero or more multi-hop route hints. Update
get_route's interface to accept such hints, although only the last hop
from each is used for the time being.
Moves RouteHint from lightning-invoice crate to lightning crate. Adds a
PrivateRoute wrapper around RouteHint for use in lightning-invoice.
While this is less readable, I spent way too long trying to adapt
the bindings generation code to handle glob imports and concluded
it would take refactoring almost the entire import-resolution
logic. While this may be a good refactor to do eventually, its
probably not worth it today.
This prevents aliasing the global secp256k1::Signature name in C
bindings and also makes it a little more explicit that the object
is different from other signature types.
