Add something like Collector to the public API

[nedbat]

Originally reported by David MacIver (Bitbucket: david_maciver_, GitHub: Unknown)

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As part of Hypothesis's new coverage guided test generation, I want to be able to run the coverage collector infrastructure without worrying about saving to the file system, etc.

I am currently using a number of APIs that are clearly internal and would prefer not to be.

I don't know that the right thing to do would be to just declare the Collector API public - it has a lot of little details that made it a bit awkward to use here (e.g. file disposition types), but perhaps it would be possible to write a small user friendly wrapper around it and make that part of the public API?

I'd be happy to try to put together a pull request for this if you're amenable.

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• Bitbucket: https://bitbucket.org/ned/coveragepy/issue/603

[nedbat]

@DRMacIver I'm not sure Hypopthesis still needs these changes. I've opened a related issue: #921