API/CLN: make rename and copy consistent across NDFrame
verifies #2721, #4039
closes #4605

TST: add tests/test_generic.py

API/CLN: Refactor rename methods to core/generic.py; fixes Series.rename for (GH4605), and adds rename
   with the same signature for `Panel`

API/CLN: Refactor Series.copy to core/generic.py; removes the order argument from Series.copy (inconsistent)
