This commit does to things: moves the prior ShortChannelID struct into
a new short_channel_id.go file, and also implements the new ChannelID’s
currently used within he specification.
These new ID’s are 32-bytes in length and used during initial channel
funding as well as during normal channel updates. During initial
channel funding, the ID is to be a random 32-byte string, while once
normal channel operation has began, the ID is to be (txid XOR index),
where index is the index of the funding outpoint.
