Since we never attempt to sweep an HTLC we offered with the preimage on
the remote's commitment, we never use the constant
AcceptedHtlcSuccessWitnessSize for weight estimation. Similarly, we
never timout an HTLC offered by the remote on our own commitment, and
don't need the constant OfferedHtlcTimeoutWitnessSize.
In this commit, we update the `CommitSpendNoDelay` method to be aware of
the alternate spending mechanism for commitments that don't have a tweak
for the remote party's non-delay output. We also add a new witness type
so callers can convey their expected signing path.
This commit is a step to split the lnwallet package. It puts the Input
interface and implementations in a separate package along with all their
dependencies from lnwallet.
