Move input to subfolder before refactor

Gonna inherit from it on two subclasses: "P2SH Input" and "P2PKH Input"

They'll have a `addSignature` method, to deal with new signatures, a `clearSignatures`, in case the outputs change and the signatures get invalidated, `countMissingSignatures`, and a reference to the UTXO they'll be spending, so all logic regarding "p2sh mapping" (redeem script info) gets to be in here and the Transaction class is not polluted.