initial work on with_trace()

[MichaelChirico]

Closes #200

Still some work to be done, but filing now for early feedback, e.g.

1. API design -- I was a bit unsure the best way to handle the signature. In particular, notice from the test that we always have to specify the trace() arguments by name, otherwise use (what, code, args), which doesn't exactly match what we're used to when using trace() directly. Open to suggestions here
2. Approach for testing. IIUC trace() basically defaults to using .GlobalEnv to look up objects to be traced, which doesn't play well with the normal approach to writing testthat tests -- as seen here, we have to write to .GlobalEnv to test the default behavior. Feels odd, but also didn't feel right to always supply the unit test environment as an argument. Again, open to suggestions -- probably best is to just add several tests and comments about why they're needed. Or we can try and diverge from the base::trace() behavior and always seek to land in the calling environment.
3. Other repo-specific nitty-gritty like, file naming, style conventions, etc. e.g. I landed this in R/debug.R with the thought that eventually we could put with_debug() here (or other temp state edits for debugging).