Nice to have: rlua::String could use Eq and Hash

[singalen]

I tried to collect rlua::Strings into a HashSet (inside a Function) and found that it has neither Eq nor Hash.
I can convert them to std::Strings, but I believe it will lead to excess copies.

[jugglerchris]

These both look like good ideas to me! Eq can just be derived, and the Hash implementation can hash String::as_bytes().