Native b58 implementation

[peerchemist]

All unittests are passing.

[SimoneBronzini]

Thanks for your PR! The code looks fine, however I believe we should have more thorough testing, base58 being a very important core part of the system. Some base58 test vectors can be found at the following links:

• Bitcoin Core vectors (these strings apparently need to be unhexlifyed before encoding and hexlifyed after decoding to be compared)
• Base58 go library (some conversions needed as above)
• These Bitcoin Core test vectors might be another good source of inspiration

One more thing: I see the test class is called P2THTest and the same is printed at test setUpClass. I guess that is the wrong name for these tests.

[peerchemist]

We've been using this b58 implementation for well over a year in the pypeerassets project. I've simply copied the code over and implemented a test in unittest (we use pytest here).

[peerchemist]

Expanded the unittest.