Steal/merge Kaushik's eager-freeze evaluator

[inducer]

https://github.com/kaushikcfd/actx_dgfem_suite/blob/main/actx_dgfem_suite/arraycontext/evaluate_pt_exprs_using_pyopencl.py

Needs the index-to-high-level-op thingy in pytato. 🤔