Implement common traits for Encoding

[freshstrangemusic]

It would be useful to have Debug, Clone, Eq/PartialEq implemented or dervied for Encoding/EncodingRef so that structs can contain an EncodingRef and have those traits be derived.

[freshstrangemusic]

Ah, so it turns out Debug is implemented in 0.3+, which is unreleased. Still, the other traits would be useful.