REF: make ops a directory
In working on #27130 I'm finding we have a number of kludges where we check for operator names instead of operator identity because we haven't imported ops.radd, ops.rsub, etc.  This is in general because of circularity-avoidance.  So this PR breaks off those function definitions into roperator.py in a new core.ops directory.

In a follow-up, parts of core.missing devoted to handling division by zero will belong in core/ops/