Addresses #96.
Turns out it was being used for hex encoding/decoding, so replaced that with the `hex` crate.
i chose to import the `decode` method as:
```
use hex::decode as hex_decode
```
so that it is clear to the reader what is being decoded when it is called. "decode" is such a generic sounding function name that it would get confusing otherwise.
This resolves an very unergonomic API by allowing iteration over a
Transaction being signed's inputs without needing to take a
conflicting reference to the transaction.
The API is still relateively unsafe in that its very easy to
generate bogus sighashes with it, but this is much better than it
was, and its not clear how to fix it further.
Jean Pierre Dudey
Savil Srivastava
Matt Corallo
Andrew Poelstra