Now:
- Combine nested maps into single ParFor 
- Extend C backend to turn ParFor/IndexReduce/IndexScan expressions into CUDA kernels 

Long term:
- Indexing by boolean masks
- Support 'output' parameter of ufuncs 
- Garbage collection 


On pause:
- Adverb semantics for conv
- Code generation for conv

Maybe never?
- Adverb-level vectorization 

Old:
- Only run tiling on perfectly nested code
