(Nearly) full Login packet implementation

[RadiatedMonkey]

This pull request implements most of the Login packet (except deserialization of the player's skin) including verification of the token signatures.

Other changes I made outside of the Login packet itself:

• ProtoCodecError::UTF8Error now uses the std::str::Utf8Error type internally rather than std::string::FromUtf8Error. Since the String error can be converted into the str one, this makes it possible to use the error variant for both str and String conversions.