A panic handler is a block to execute when a process panics. Once the
block finishes running, the process terminates. A panic handler can be
registered using `std::process.panicking`:

    import std::process
    import std::stdio::stderr
    import std::vm

    process.panicking do (error) {
      stderr.print(error)
    }

    vm.panic('oops!')

Each process can only register a single panic handler, and newly
registered handlers will overwrite any previous ones.

The block passed to `std::process.panicking` is given the panic message.
A stacktrace needs to be obtained manually using
`std::debug.stacktrace`.