createCommitmentTx would earlier mutate the passed commitment struct
after evaluating the htlc view and calculating the final balances, which
was confusing since the balances are supposed to only be *after*
subtracting fees.
Instead we take the needed parameters as arguments, and return the final
balances, tx and fee to populate the commitment struct in a proper way.
PURE CODE MOVE:
Moving createCommitmentTx, CreateCommitTx, createStateHintObfuscator,
CommitmentKeyRing, DeriveCommitmentKeys, addHTLC, genHtlcScripts
We move the methods and structs to a new file commitment.go in
preparation for defining all the logic that is dependent on the channel
type in this new file.
