Merge: Introducing the do ... catch ... end structure

Merge: Introducing the do ... catch ... end structure

This PR is a first step in trying to handle exceptions in Nit, replacing the behaviour of `abort` if it happens within a do ... catch ... end.

In the compiler, setjmp() et longjmp() are used to jump directly from the `abort` to the nearest `catch` bloc.

Pull-Request: #2011
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>