Port to new pymbolic

[alexfikl]

Updates things to use expr_dataclass and fixes various deprecation warnings.

(Still WIP, but thought to throw it in here anyway)

Needs:

• Add explicit scopes to CSE pymbolic#150
• Remove some deprecations from dataclass Expressions sumpy#217
• Pymbolic typing loopy#868

[alexfikl]

This seems to be in a better state than expected! The remaining deprecation warnings are from loopy and sumpy and it just need a little change in pymbolic for __eq__ in IntG to work.