Parallel pytato PyOpenCL array context

[majosm]

Adds PytatoParallelPyOpenCLArrayContext, which is like PytatoPyOpenCLArrayContext but parallelizes over the device. Based on @kaushikcfd's work in #216. This version generalizes that code to work with fused loops.

The basic procedure is:

1. Collect instructions that share inames into sets.
2. Find the "outer" inames (i.e., inames shared between all instructions) in each of those sets.
3. Parallelize over those.

cc @lukeolson

⚠️ Currently WIP. TODO:

• Figure out why some tests are failing (here and in Parallelization transformation loopy#1010)
• Re-run profiling on Tuolumne to make sure I haven't messed anything up
• Merge loopy deps
• Add docs