Base16 and Base64 for PicoRuby

[sylph01]

I've implemented Base16 (equivalent of pack('H*) / unpack1('H*')) and Base64.
I will be using them to test cryptographic functions that are going to be added into the MbedTLS gem. This might be of general interest when networking functionality comes to PicoRuby.

Base64's interface is pretty much the same as CRuby, except it does not include line breaks.
Base16's interface is built to look like Base64's, so it works like this:

irb> s = Base16.encode16 'hogefugapiyo'
=> "686f6765667567617069796f"
irb> Base16.decode16 s
=> "hogefugapiyo"