This converts the txsort code into a separate package and renames
'TxSort' and 'IsTxSorted' to 'Sort' an `IsSorted`, respectively.
It also adds a 'doc.go' and 'README.md' so it is consistent with the
other packages.
These functions sort transaction inputs and outputs according to BIP LI01
(https://github.com/kristovatlas/rfc/blob/master/bips/bip-li01.mediawiki)
This is useful to standardize transactions for faster multi-party
agreement as well as preventing information leaks in a single-party
use case.