Public broadcast wrapper and recursively_apply tool

[nsmith-]

Both functions in awkward1._util are used a few times in coffea. The wrapper function that broadcasts and flattens inputs for numpy ufuncs was copied in a simplified form to coffea lookup_tools: https://github.com/CoffeaTeam/coffea/blob/master/coffea/lookup_tools/lookup_base.py#L19-L45 The recursively_apply function is used to make sure a index take operates on the flat array in NanoEvents cross-references.