pw_allocator: Add a benchmark test harness

This CL enables collecting benchmarks that can be used to evaluate
improvements to performance, overhead, and/or fragmenetaion of
subsequent CLs.

The benchmarks can be collected by running the benchmark.py script,
which generates a CSV file that can be added to
go/pw_allocator-benchmarks.

Change-Id: I83e7311fc5a993f14d8d1180a718a530f54d5c56
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/231331
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Commit-Queue: Aaron Green <aarongreen@google.com>
Reviewed-by: Taylor Cramer <cramertj@google.com>
Docs-Not-Needed: Aaron Green <aarongreen@google.com>