Fix EncodeHex/DecodeHex

[dolmen]

Fixes for the unfortunately merged #41:

• add testcase for encoding (the testcase in Hex #41 only checks roundtrip but this doesn't enforce compatibility with other implementations or protects against change in the encoding)
• check bounds of decoded numbers in DecodeHex (to avoid crashing)
• faster EncodeHex
• faster DecodeHex
• add documentation about the encoding (important as it is quite awkward: each hex digit is encoded as an int in range [16, 31])

Note: this is not an endorsement of the EncodeHex/DecodeHex feature. Instead I would prefer it would be completely removed because it is quite awkward.

Cc: @brandoneprice31

[speps]

Thanks a lot.

[speps]

I agree it should ideally go. As I said in the other thread, let me know if you want more rights on this repo and get it to 2.0 (breaking changes, remove fluff, etc.).