This commits makes use of source maps and the original source
when printing lacking line coverage in the pretty printer.
Only the executable lines are checked as before (as non-executable
lines will always be ignored anyways). The lines then mapped to the
appropriate source line when a source map is present.
Merging multiple runs isn't quite right because we
rely on a 0 count to signal that a block hasn't been called.
Other tools like c8 expect this to be true as-well so we
need to do our best to merge coverage files rather
than duplicating them.
