Find executable code for coverage analysis

[Pchelolo]

We want to be able to find all "executable" lines in the hack codebase in order to compute test coverage.

This sets up the backbone for the new command, runs a visitor over all files in the codebase and outputs the results in a file. Right now we output start and end of each statement. It's not perfect, but I'll make adjustments in followups.

[salesforce-cla]

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[muglug]

This looks good! You might want to look at the test file config to only generate for production files

[salesforce-cla]

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[Pchelolo]

This looks good! You might want to look at the test file config to only generate for production files

Couldn't really figure out how to do that. Will try in a followup - there's still work to be done to properly classify various things as covered/non-covered. But we'd like to release this to try it out in read CI and make bug finding more collaborative.