Standard library `_operator` module is poorly typed

The annotations for functions in the _operator module are inconsistent - some have annotations but most don't. At first glance, most of the protocols necessary to type these incomplete signatures can be imported from _typeshed, and there is already a precedent in [the stub in question](https://github.com/python/typeshed/blob/72b73680a27b8c92e6fdc6886c2f20448b273a36/stdlib/_operator.pyi#L2). I can work on the annotations to bridge the inconsistency, but it would be a big undertaking, so I need to get approval first.