Add a feature for using the same number of loops as a previous run

[mdboom]

Motivation:

On the Faster CPython team, we often collect pystats (counters of various interpreter events) by running the benchmark suite. It is very useful to compare the stats between two commits to see how a pull request affects the interpreter. Unfortunately, with pyperformance's default behavior where the number of loops is automatically calibrated, each benchmark may not be run the same number of times from run-to-run, making the data hard to compare.

This change adds a new argument to the run command which will use the same number of loops as a previous run. The loops for each benchmark is looked up from the metadata in the .json output of that previous run, and passed to the underlying call to pyperf using the --loops argument.

Additionally, this modifies one of the benchmarks, sqlglot to be compatible with that scheme. sqlglot is the only run_benchmark.py script that runs multiple benchmarks within it in a single call to the script. This makes it impossible to set the number of loops independently for each of these benchmarks. It's been updated to use the pattern from other "suites" of benchmarks (e.g. async_tree) where each benchmark has its own .toml file and is run independently. This should still be backward compatible with older data collected from this benchmark, but doing pyperformance run -b sqlglot will now only run a single benchmark.

[Fidget-Spinner]

Looks good in general. I'll let Eric approve though.

[ericsnowcurrently]

LGTM

I left a couple of minor comments that you can take or leave.

It might be worth splitting out the bm_sqlglot changes into a separate PR, but it isn't a big deal. I'll leave that up to you.

[mdboom]

@ericsnowcurrently: I think this is good to merge, but I don't have merge rights, if you don't mind... :)