Use std::backtrace instead of backtrace crate
Wraps the Backtrace in an Arc, since it is not Clone.
Currently uses Backtrace::force_capture
to ignore the RUST_BACKTRACE
environment variable.
Removes resolve_backtraces from rtmock since it is no longer needed as std's backtraces lazily resolve without needing a &mut.
Closes #1144 (closed)
Edited by Robin Leander Schröder