Update benchmark sampling method and statistics

[popematt]

Issue #, if available:

While not necessarily a fix, this does make significant progress on #265

Description of changes:

• Updates/fixes the sampling methodology so that it's properly running samples of 1-2 seconds each.
• Adds a SampleDist class that helps compute statistics for a sample distribution
• Drops the "rate" metrics and adds operations per second metrics.
• Drops the percentile stats and adds error and standard deviation.
• Refactors the report generator to make it less spaghetti-like
• Updates the table output format so that it can be directly copy-pasted to github as a markdown table.
• Updates the compare command so that the fields argument uses the same field names and syntax as the other commands.
• Drops support for Python 3.7 (which is EOL) because the statistics.NormalDist class requires at least Python 3.8.

The perf testing workflow is expected to fail as I have not updated that. It will come in the next PR.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.