[Chore] Cleanup parse_data_from_io legacy/segwit parsing

[zquestz]

This just cleans up the parse_data_from_io method and makes it more readable.

I am new to this gem, so let me know if I messed anything up. ;)

[etscrivner]

LGTM

[zquestz]

Ok all tests are now passing. Fixed the issue.

Docs: https://bitcoin.org/en/developer-reference#raw-transaction-format https://github.com/bitcoin/bips/blob/master/bip-0144.mediawiki

[lian]

cherry-picked into master