Update rnn benchmarks
RNN benchmarks can be run with python rnns/runner.py which runs
everything and gives a nice summary view.
They are written in the style of the benchmarks in the scripts/ folder
(those are used by the CI).
Run individual benchmarks with (cd into ./rnns):
python -m benchmarks.sru
Individuals scripts have their own command line options to test
different things (--jit, --backward, etc).
