use Cows again

[fiji-flo]

First of all great PR!

Since a major part of the work in rari is moving strings around I tried to allocate as little as necessary. When I changed most strings to Cows it really made a difference in sum.

That's why there's Cows everywhere. I'm thinking about making this more ergonomic in the future and I will definitely add this in a contributing guide.

That's the difference in this PR doing a simple get_group("EvalError", &[]):

Running benches/jsref.rs (target/release/deps/jsref-9250feaa2ada967f)
jsref EvalError         time:   [84.634 ns 85.175 ns 85.753 ns]

Running benches/jsref.rs (target/release/deps/jsref-9250feaa2ada967f)
jsref EvalError         time:   [72.245 ns 72.899 ns 73.573 ns]
                        change: [-16.171% -15.542% -14.890%] (p = 0.00 < 0.05)
                        Performance has improved.

Feel free to ping me if you want to chat about this.

[Josh-Cena]

OK, the profiling looks convincing. I wouldn't sacrifice a fraction of a millisecond for more readability, but the optimization makes sense.