mpz-memory: first check if allocated, then if set

[themighty1]

Currently the VisibilityView API allow this test to pass

#[test]
    fn test() {
        let mut view = VisibilityView::new();
        view.alloc(10);
        assert!(view.is_set(20..30));
    }

It obviously shouldn't pass since the range 20..30 was not allocated.

Is the caller responsible for passing only the ranges which were allocated or is this a bug that needs fixing @sinui0

[sinui0]

yes, this should probably be regarded as a bug

[themighty1]

Im realizing this requires a decision how to treat access to unallocated memory without adding an error to fn sig. We may also want to go back to using Slices instead of Ranges.
Was there a reason why we don't use Slices anymore for these APIs? @sinui0