Add a std::panic::propagate function to pair with std::panic::recover

Say we're integrating with a C API where the control flow will pass from Rust to C and then back into Rust. If the inner Rust code panics, we have to stop that panic from unwinding through the C layer. std::panic::recover will allow us to do this. But now we would ideally want to carry the panic payload through the C layer and "restart" unwinding when we're back in Rust-land. We can just panic!(payload), but that will rerun the panic handler function, printing the "thread foo panicked" an extra time. This situation becomes a bit more unfortunate with custom panic handlers, since those handlers may do more expensive tasks than just printing a message.

To solve this, we could add a function which acts like panic!() but does not run the panic hander. Since we already have the payload, a normal function taking a Box<Any + Send> will suffice rather than a macro.

Start with the issue's description of std::panic::recover and the proposed std::panic::propagate API, then inspect the Rust standard-library panic implementation. Done means carrying a Box<Any + Send> across a C boundary and resuming unwinding without invoking the panic handler a second time.