Use parse::<Txid>() in place of encode::deserialize_hex() [issue 11]

[fedemagnani]

Closes #11

[apoelstra]

ACK f2f891b; successfully ran local tests; interesting about the changed error type

[tcharding]

This is awesome; I had not imagined we would get rid of the encode::FromHex error returns - that is cool. Thanks

Can I be a pest and ask you to write a more complete commit log please

Something like

Use parse::<Txid> instead of encode::deserialize_hex

Currently, in some places, we use the bespoke `encode::deserialize_hex` function to deserialize txid from a hex string. The `Txid` type provides a `FromStr` impl so we can use `parse::<Txid>()`. This makes the code more easily readable because its a common pattern in Rust.

(If you cut'n'past don't use a single line for the body, max column width 72 please.)

Thanks man.

If you are interested in git commit logs check out the very educational post: https://cbea.ms/git-commit/

[fedemagnani]

Thank you guys. I amended the commit including the message sugested. Does it look good? @tcharding