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, as it should be, restricts OnionHopData to only being able to
represent valid states, while still allowing for tests to generate
bogus hop data fields to test deserialization.
Its a bit awkward to have an hmac field covering the struct that
its in, and there is little difference in removing it, so just pull
it out and use a [u8; 32] where we care about the hmac.
Previously OnionHopData contained a OnionRealm0HopData field however
instead of bumping the realm number, it has been replaced with a
length, used to indicte the length of a TLV-formatted object.
Because a TLV-formatted hop data can contain the same information as
a realm-0 hop data, we flatten the field and simply keep track of
what format it was in.
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.
