Investigate a Zero-Copy interface

Currently, this crate forces the use of many allocations.

It should be possible to make Value use Cow internally, as well as making Request store an &'a [Value] for the arguments (note that the name of the called method is already a &'a str). This makes adding single arguments impossible, but that's not really a problem since the user can just use a Vec themselves.

One thing to note is that this will probably make Value a bit clunkier to use: Cow is hard to match on. Also, I've noticed that implementing this causes a cyclic trait dependency.