Fix state passthrough (banish the shadow realm)

[Alex-Fischman]

This PR fixes the state edge passthrough performance bug by "tying the knot" so that loop contexts point to the loop itself (instead of to the pre-context version of the loop). To this end, it also:

• Refactors add_context.rs
• Refactors the schedule
• Changes the code that builds the program string in dag_in_context/src/lib.rs
• Touches almost every test in the dag_in_context subcrate to add the unions to the egglog_test
• Because we need to create temporary variables in add_context.rs, and because substitution adds context, we have to pass a ContextCache throughout our code.
• Deletes the empty fuction_inlining.egg file
• Fixes the redundant switch rewrite rules and moves them to passthrough
• Improves the debug-helper.egg file
• Does some other stuff too, like adding two new snapshot tests

[ajpal]

I'd like to see a solid nightly run before we merge, but code lgtm!

[Alex-Fischman]

I just looked at eggcc-notify and the nightly for main has been failing since May 31st. Since I rebased my branch is also failing. I don't know why.

[Alex-Fischman]

Rebased on fix-install (#623).