We'll need to pack feature vectors for route blinding, so we pull
the encoding/decoding out into separate functions (currently
contained in ChannelType). Though it's more lines of code, we keep
most of the ChannelType assertions so that we strictly enforce
use of the alias.
In this commit, we add a new TLV record that's intended to be used as an
explicit channel commitment type for a new form of funding negotiation,
and later on a dynamic commitment upgrade protocol. As defined, we have
3 channel types: base (the OG), tweakless, and anchors w/ zero fee
HTLCs. We omit the original variant of anchors as it was never truly
deployed from the PoV of lnd.
