Fix existing bug with multibase ToCid

[austinabell]

• Refactored and added tests for existing ToCid code block. (I want to do a complete rewrite of this in the future, since this trait is not necessary and isn't well setup imo or tested, but not a priority)
• Added string decoding test to make sure version and codec we are using will be decodable when needed

decoding from string to bytes is broken currently because multibase 0.6 is broken. When updating to 0.7 they removed at least lower case base32 so I wrote a test that currently panics because it isn't supported (but just throws a parse error lol) and will either open an issue or add support for other encoding types in multibase and update at a later time.

There is no rush on this because we will be interacting with bytes, but will document now for when it gets to that point.

Edit: The rewrite I'm referencing by the way for ToCid is to just use the TryFrom trait as this is exactly the same as the trait they setup and is more usable: https://doc.rust-lang.org/std/convert/trait.TryFrom.html