Initial base64 implementation

[jtenner]

Another stab at a base64 implementation provided by https://github.com/lemire/fastbase64/blob/master/src/chromiumbase64.c

Not sure what the right way to use this software is, but we should probably use their license, and their tests to make sure everything works as intended.

• The implementation requires a lot of data, is there any way to make this smaller?
• What other tests should we include