Implement gcov coverage provider

[dbarnett]

Add a coverage provider to support reading gcov coverage results into vim.

[okkays]

I can take a look at this 👍

[dbarnett]

BTW, are we calling the provider "lcov" or "gcov"? IIUC gcov is the technology and lcov is a format & frontend for dumping the gcov results for viewing? If so, slight preference for "gcov" as a more general provider name.

[okkays]

👍 makes sense, I'll rename in the pull to gcov. I went with lcov because that's the format it's actually parsing, but using the more general gcov makes more sense for discovery