TYPES: add types in core.util.hashing
cc @simonjayhawkins 
@simonjayhawkins changing np.uint64 to int caused mypy to complain.  any other suggestions? Shouldn't the return type of `_combine_hash_arrays` be an ndarray? The annotation says `int` but mypy is complaining because on line 210 we are trying to index what `_combine_hash_arrays` returns > Shouldn't the return type of _combine_hash_arrays be an ndarray

I think you're right, yes.  Will update.  I look forward to the day when annotations support cython-like `ndarray[uint64, ndim=1]` or something. Closing for now to clear the queue, will troubleshoot locally.