Since [0], removing degenerate points at the beginning of the hull
would re-order points so the last were moved to the beginning.
While this isn't an error, having the resulting hull *sometimes*
re-ordering it's result based on internal error correction isn't ideal.
Document that the first point in the hull has the lowest Y value and
update tests to ensure this.
Also correct the doc-string regarding the hulls cross-product
and tests this is working as documented.
[0]: 87f9fd8fb3
32f9b65cc5