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.
Rather than having methods taking &mut self, have them consume self
and return another Builder, so that methods can be chained.
Bump major version number.
Does not do stuff like validating the form of contracts, since this seems like
more of an application thing. Does not even distinguish a "nonce", just assumes
the contract has whatever uniqueness is needed baked in.