BIPLI01 has been accepted as BIP0069, so update the README, comments, tests, and testdata files as such.
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.