Write benchmark tests for common transforms/queries

I found this section about benchmark tests in the Rust docs and thought it might be relevant to write some for tests in aw-transforms or aw-query: https://doc.rust-lang.org/1.7.0/book/benchmark-tests.html

There's also this, which runs the benchmark tests on two commits and compares the results: https://github.com/BurntSushi/cargo-benchcmp

Would be interesting to use for comparing different releases or comparing different allocators (#144).

Have you looked at this before @johan-bjareholt?