Support for mixed R and native C/C++ profiling

[atheriel]

There is an early attempt to do this using libunwind in a branch of the same name. It did work under some limited conditions, but consistently merging the two stacks together will require much more serious thought. libunwind will likely not work on Windows (#3), either.

Ben Frederickson (of the py-spy project) has an excellent blog post about that project's approach to the same problem, albeit in Rust.

This would also pave the way to this project being used as a backend for the jointprof package.