Implement `delaunay!`, in-place version of `delaunay`

Now, `delaunay` allocates the output. It would be nice to have the corresponding in-place version `delaunay!`, where the user provides the output. A part of performance reasons, the in-place version would allow the user to select the container type (as long as it has the correct memory layout).

Once `delaunay!` is available, `delaunay` can be implemented by calling `delaunay!`.

Start by reading the existing `delaunay` implementation and its surrounding API to understand how output is allocated and represented. Add a `delaunay!` entry point that accepts a caller-provided container with the required memory layout, then make `delaunay` use it while preserving its current behavior.

Written by the indexing model from the issue text.