Use RouteHop's new node_features to send TLV-encoded onion hops

Use RouteHop's new node_features to send TLV-encoded onion hops

This implements the new TLV variable-length encoding for onion hop
data, opting to send it if the RouteHop's node_features indicates
support. It also uses the new process_inline method in ChaCha20 to
optimize a few things (though it grows a new TODO for a
probably-important optimization).