Include performance results in JSON output

[nobodywasishere]

Feature Request

It would be useful to have a way of automatically store compiler performance metrics over time, to see how a projects build times and module count change as changes are made to the codebase. This may also help setup something along the lines of https://perf.rust-lang.org/.

This could be accomplished by when using both the -s and -f json flags are used, the stats output is included in the JSON (it's instead printed to stdout iirc).

This would break anything not setup to handle new fields being added to the existing json.