Add allocator benchmarks + more benchmark options

[llbit]

Added two new options to the benchmark script:

• --timestamp allows manually specifying the timestamp for the benchmarked version.
• --no-build skips building Rho before running a benchmark. Useful when re-running the benchmark for a single version.

This also adds new benchmarks for benchmarking the Rho allocator.

[codecov-io]

Current coverage is 70.08% (diff: 100%)

Merging #110 into master will not change coverage

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