Change EvalExpr.evalute to return Cows to avoid unnecessary clones (#291)

Refactors `EvalExpr`'s `evaluate` function to return a [`Cow`](https://doc.rust-lang.org/std/borrow/enum.Cow.html#).

Expressions like paths and variable reference return a `Cow::Borrowed` value and value-generating expressions (e.g., `+`, `trim`, ||`, etc) return a `Cow::Owned` value.

Locally, this results in a ~15% to ~30% speedup on evaluation benchmarks and ~40% speedup of the run of the entire conformance test suite.

Author: jpschorr