Add compile-time coverage for compiled code

[sklam]

Let Lower gather coverage during compilation and write to active Coverage object.

It will use "numba_compiled" context to mark the lines visited by the compiler:

[tasansal]

Looking forward to this! Thanks for working on it! Will this make our numba code coverage on downstream libraries as well?

[sklam]

Note that this is code coverage reported during compile-time, and not runtime code coverage, which I still cannot find a efficient way to do.

Will this make our numba code coverage on downstream libraries as well?

Yes, it works as long as the Numba can see an active coverage.Coverage object during compilation.