Raise implementation for
RubyException boxed trait objects.
This function unwinds the stack with
longjmp, which will ignore all Rust
landing pads for panics and exit routines for cleaning up borrows. Callers
should ensure that only
Copy items are alive in the current stack frame.
Because this precondition must hold for all frames between the caller and
sys::mrb_protect landing pad, this function should only be
called in the entry point into Rust from mruby.