Add ternary operator

[raffopazzo]

This seems particularly useful to construct types.

func range_up  (u64_t from, u64_t to) -> array_t(u64_t, from < to ? to - from : 0);
func range_down(u64_t from, u64_t to) -> array_t(u64_t, from > to ? from - to : 0);

Also, the returned value might be a dependent type

func zero(bool_t which) -> which ? i32_t : bool_t
{
    return which ? 0 : false;
}

But this is not really surprising because all functional languages have the expression if x then y else z .