Rename "struct arrfunc" to "struct arrfunc_type_data"
Switch the arrfunc to use a funcproto instead of a list of types
Simplify arrfunc_from_pyfunc, rename to arrfunc_from_instantiate_pyfunc
_lowlevel.arrfunc_from_pyfunc, creates an arrfunc from a python function
Introduce nd::arrfunc to hold arrfunc objects
Merge pull request #83 from mwiebe/rolling_apply

Work towards being able to do rolling apply