Update & refactor json-b64 crate

[tgross35]

• Json, Bytes, and Secret are now exported at top level
• Rename Bytes to B64Bytes and Secret to B64Secret to better reflect their purpose (this is optional, can revert if desired)
• Add example to readme
• Rename internal modules for clarity

This somewhat improves the usability of the b64 crate. It is a breaking change, but I don't think many people are using this crate as-is.

[tarcieri]

@tgross35 can you rebase? That should fix the test failure