Proposal: let's remove xxx_insert versions of algorithms taking OutputIterator

The algorithms:
- `convex_hull_insert`
- `intersection_insert`
- `difference_insert`
- `simplify_insert`
- `sym_difference_insert`
- `union_insert`

The main reason is that they take nonintuitive parameters, e.g. `intersection_insert` takes OutputIterator and in addition to this SingleGeometry because an iterator doesn't hold the necessary information for the algorithm to work. Another example is `convex_hull_insert` which takes the compile-time information Closure and Order from the input Geometry because the iterator doesn't hold it which can result e.g. in taking it from a Box while filling some unrelated output Range/Ring/Polygon with points.

Furthermore, they've been hidden in `detail` namespace so probably nobody uses them. But they have to be maintained.

What do you think?