CL 491875 introduces a new bisect command, which we plan to
document for use by end users to debug semantic changes in
the compiler and in GODEBUGs.
This CL adds bisect support to GODEBUGs, at least the ones
used via internal/godebug. Support for runtime-internal
GODEBUGs like panicnil will take a bit more work in followup CLs.
The new API in internal/bisect to support stack-based bisecting
should be easily reusable in non-GODEBUG settings as well,
once we finalize and export the API.