Base64 encoding

I see that there is an option for hex encoding via .toJSON() or .toString(16), but how about base64 encoding - is that supported at all? Is it possible to extract the bytes in a form that may be passed to `btoa()`?

Start by inspecting the existing .toJSON() and .toString(16) encoding behavior in bn.js, then check how callers could obtain bytes for btoa(). Done when the issue's question about base64 support and byte extraction is answered by tested, documented behavior.

Written by the indexing model from the issue text.