[RFC][Tx Sort] Implement sorting of inputs

[RFC][Tx Sort] Implement sorting of inputs

Follows BIP69: https://github.com/bitcoin/bips/blob/master/bip-0069.mediawiki

1. Implements sorting of transactoin inputs.
- BIP says to use "reversed byte-order" for the `prev_hash`. I interpreted this as: little-endian.
- TODO need to add tests

2. Re: improve sorting of TxOut's script_pubkey to use lexicographic ordering, and not length.
From the test-cases i've included it seems that the current code already does lexicographic ordering (and not length based). Am i missing something?