libu8 contours fix

[haase]

This is really a libu8 bug, but the best place to demonstrate it is by signalling a Scheme error. It looks like u8_throw_contour goes to the next contour, above the innermost one, while it should really longjmp to the current contour, which should then pop, potentially copying an error from the contour.

To explore, define (oops x) to generate an error. break on the u8_do_contour functions and evaluate (oops).

[haase]

This has been fixed in libu8 and the references to it in KNO have also been updated