Add asv benchmark for compression ratio of some example VCFs

[tomwhite]

Suggested by @benjeffery in https://github.com/pystatgen/sgkit/pull/937#pullrequestreview-1145718679

[tomwhite]

@hammer notes that https://github.com/ionelmc/pytest-benchmark or https://github.com/conbench/conbench are alternatives to ASV.

[hammer]

I was also pointed to airspeed-velocity/asv#1219 as an important issue where the future of ASV is being discussed.

[hammer]

It definitely sounds like pytest-benchmark is the simpler starting point.

[hammer]

Nvidia made a tool to transform pytest-benchmark output to a format expected by asv for visualization: https://github.com/rapidsai/asvdb

[tomwhite]

It seems that pytest-benchmark doesn't support custom metrics that aren't timing-related, i.e. compression ratio in our case here.

coiled/benchmarks#186 is worth a read.

[tomwhite]

Fixed in #978