Fix NegativeArrayException on 018_heap

[JC626]

In 018_heap/main a NegativeArrayException can occur for:

type Heap is ({
    int[] data,
    int length
} h)

This is due to the length of the Heap being less than zero.
The length of the Heap could also be outside of the bounds of the array, data.
Therefore, type constraints would need to be added to ensure the length is within the array's bounds.

[DavePearce]

Nice!