Idea: function contracts + CTFE

[dennis luehring]

> Sounds totally unnecessary.
> if you pass 30 to the above fork you already know that it's not between
> 0 and 20 (that is if you finished 2nd grade of school). 

just think of x 100 000 lines of code projects - not these 3 lines 
examples - and think of projects that are growing over years ...

 > you need help
> from the compiler when you do not know what x is, i.e. in run-time.
> I don't see any practical benefits to this outside of meta-programming
> and there I'd expect the compiler to throw the assert error during
> compilation.

why should the meta-programming section be more "safe" than normal 
coding? both sides can benefit