Introduce ReductionOperation class, accept 'initial' in reductions

[inducer]

This replaces the strings we used to use to identify reductions with something a bit more structured. The impulse for this came from inducer/arraycontext#129, which needs to supply initial to array reductions to match the numpy interface for empty min/max reductions. This only adds very minimal support for supplying initial, in that it allows the neutral element to be passed (and obtaining that is what led to the structured reduction op types), and it better mimics numpy's behavior for empty reductions.

cc @majosm

[kaushikcfd]

This almost looks great! Found some minor numpy compat issues, feel free to merge after that.

Thanks!

[kaushikcfd]

Should this be _NoValue as well? Numpy does it, see https://numpy.org/doc/stable/reference/generated/numpy.prod.html#numpy.prod.

[inducer]

I think this is OK. Numpy will return 1 for an empty prod. I think for an immutable object like 1 it does not matter whether that's done via _NoValue and then setting internally, or directly like this.

[kaushikcfd]

Found out some more places where the implementation disagrees Numpy, good to go after that.