Remove various Windows assumptions (.bat files, CMD, etc) in solution

[kevin-montrose]

I'm on Windows, but it's 2020 and .NET doesn't only run on Windows.

Change all the .bat files to be cross platform Powershell scripts:

• GenerateDocs.bat
• RunCodeCoverage.bat

As of 0.7.0 the benchmark project also makes some Windows-y assumptions (CMD being a thing, basically) that should also be excised.

• Cesil.Benchmark runs on not-Windows