Lacecore

- GroupMap: Fix `defragment()`.

- GroupMap: Add `defragment()` and `to_dict()` methods.

- `check_arity()` and `check_indices()`: Add to public interface.

- Improve `.sliced_by_plane()`:
- Slice a submesh by passing `only_for_selection`
- Slice by several planes at once
- Preserve `face_groups` when slicing
- Selection: Add `generate_masks()`method
- Selection: Improve error message for `.pick_face_groups()` on meshes without
face groups.
- Upgrade to polliwog >= 2.1.0.

- Add `.face_normals()` method.

- Mesh constructor requires face indices to have dtype `np.int64`.
- Add `lacecore.FACE_DTYPE` property.
- Upgrade to pollliwog >= 2.0.0.

While this restriction may be a little inconvenient for the caller, it improves
interoperability and performance, simplifies the implementation, and produces
more predictable return values. It's recommended that applications using lacecore
store all face indices using this dtype.