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).
This exposes the latest Init-context features in the ChannelDetails
passed to the Router during route calculation, which combines those
with the Node-context features tracked from node_announcements to
provide the latest Node-context features in RouteHop structs.
Fields are also added for Channel-context features, though those are
only partially used since no such features are defined today anyway.
These will be useful when determining whether to use new
TLV-formatted onion hop datas when generating onions for peers.
