This commit modifies the NP2WKH and NP2WSH input tests to ensure the
input count is properly incremented and accounted for in the size
estimate. 253 is chosen because it is the lowest value that, when
serialized, occupies more than one byte on the wire.
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.
