Typecast `union` as `void` to a-void the "C struct/union cannot be declared cpdef" error.

This avoids potential SEGFAULTs from Python calls

From `find` and `union` of `DisjointSet_of_hashables`, respectively

Remove `_find` & `_union`.
Remove `DisjointSet_of_integers` `_d` attribute from `DisjointSet_of_hashables` (and adjust code).
Add notes recommending `OP_find` and `OP_union` if no input checking needed.
Touch-up docstrings.