In this commit, we add the initial implementation of the SCB structure.
Given an SCB, and a user's seed, it will be possible to recover the
settled balanced of a channel in the event of total or partial data
loss. The SCB contains all information required to initiate the data
loss protection protocol once we restore the channel and connect to the
remote channel peer.
The primary way outside callers will interact with this package are via
the Pack and Unpack methods. Packing means writing a
serialized+encrypted version of the SCB to an io.Writer. Unpacking does
the opposite.
The encoding format itself uses the same encoding as we do on the wire
within Lightning. Each encoded backup begins with a version so we can
easily add or modify the serialization format in the future, if new
channel types appear, or we need to add/remove fields.
