REF: move roperators to pandas.core
This PR does 2 things:

- change `pandas.core.ops.roperator` imports to all import the module and use `roperator.<rop>` instead of importing the actual functions (this makes the usage similar as for the non-reversed operators with `import operator`)
- move `roperator.py` from pandas/core/ops to pandas/core, so it can be imported in `pandas.core.computation.expressions` without relying on the `pandas.core.ops` module, which in its turn means we can move the inline import of `pandas.core.computation.expressions` in `pandas.core.ops.array_ops` to a top-level import (better for performance for a function that is called repeatedly)

I think the first change is useful anyway (can also do it as a separate PR), for the second point, a possible alternative would be to move `pandas.core.computation.expressions` from the computation/ module to the ops/ module

One consequence of this, though: by always importing this in `array_ops`, it gets imported on `import pandas` time, and thus `numexpr` always gets imported at that point (while now it's only a delayed import when doing a first operation). 

Now, I checked with `python -X importtime -c "import numpy, pandas, numexpr"`, and after already having import numpy (and pandas), also import numexpr is only a small addition (most of the import of numexpr is import numpy). 
Based on running that a few times, it seems to adds 1.5-2 % to the total pandas import time.  