Implement SplitPytatoArrayContext that attempts a trivial parallelization strategy

[kaushikcfd]

• [x] CI should pass once https://github.com/inducer/loopy/pull/738 is in.
• [x] :warning: Concurrency across reductions is not targeted i.e. results in poor performance for computing error norms, etc. (Maybe we should fix this before merging?)
  • In the meshmode version, we desperately froze such reductions eagerly
  • Update: Added a transformation specifically for "reduce-to-scalar" operations to parallelize the reduction operation using loopy transformations.
• [ ] We need a better name than SplitPytatoArrayContext.

[kaushikcfd]

Making this a draft until the CI is fixed.