Pretty backtraces on linux

[ktoso]

Now that we don't capture any signals, and crash hard on faults (due to the #51 removal), we'd be left blind on some faults; we at least should use the swift-backtrace to get nice backtraces.
The upside is that Ian's work nicely symbolicates/demangles as well; in a nicer way than our previous PoCs did.

As Swift evolution discussions continue, the lib will adopt the APIs and we can hopefully rely on it as-is. If we'd capture faults we'd need more from it though.

[ktoso]

Marking as solved for now, we're relying on swift-backtrace now.