Add PyObject.to_int

[sklam]

This PR implements py-to-int and PyObject.to_int so that a Python int stored as PyObject can be converted into a i64.

Reason:

I am exploring (or maybe i'm abusing) the PyObject API to store Python objects into the EGraph and allowing them to generate equivalances in Python code back into EGraph. See example gist: https://gist.github.com/sklam/f141efe69bf86182ed185274b6648e93

[codspeed-hq]

CodSpeed Performance Report

Merging #393 will not alter performance

_{Comparing sklam:enh/py_to_int (17c7198) with main (6a892ec)}

Summary

✅ 14 untouched