Added keyword frame to be used for interacting with a reified version of
the current frame. This could potentially be used to simulate
traditional exceptions by something like the following, where the
prototype of frame is extended with a default handle method that simply
"upwards" the exception:

try {
    attempt
} catch {
    handler
}

=>

frame.handle = {
    handler
};
attempt

Another example:

try {
    attempt
} catch {
    throw;
}

=>

frame.handle = {
    frame.outer().handle(:exception)
};
attempt

Perhaps, a continuation could be supplied as follows:

frame.handle = {|exception, continuation|
	// Cleanup stuff, then try again
	continuation.rewind();
};
attempt