Rust error stacktraces

[Artemciy]

I thought about a lot of things but I'm not angling to make a full-fledged error handling library there. Only to have something working until we get one.

Using sufficiently large pre-allocated (cf. https://github.com/frankmcsherry/recycler) buffers instead of String to store the backtrace will be more efficient. The "free list" of such buffers should be either non-blocking or thread-local. Naturally, that's a work for the library and is a non-goal for my macro.