Refactor and consolidate the performance tests

Run perf tests by running make in test/perf
Factor out timing code into test/perf/perfutil.jl
Micro benchmarks are now in test/perf/micro/
perf2 benchmarks are now in test/perf/kernel
shootout benchmarks now run (not all yet) as part of the perf tests (#660)
cat benchmarks now run as part of the perf tests