Support statement coverage

I think the Node.js coverage reporter should include support for reporting statement coverage. My idea is to parse the source code into an Abstract Syntax Tree (AST) using `acorn-walk`, and then use the `Statement` callback to extract statements. They could then be converted to `CoverageStatement`s, (similarly to how the `getLines` function works with lines to `CoverageLines`). These statements could then be mapped to ranges in the same way that lines are handled in `mapRangeToLines`. I saw a (extremely complicated) way of doing this in <https://github.com/cenfun/monocart-coverage-reports/>, so it appears to be possible.