Add RVSDG subst functions

[rtjoa]

Summary

Adds the following functions, for use in writing passes over RVSDG

;;                   e  [ x -> v ]
(function SubstExpr (Expr i64 Operand) Expr)
(function SubstOperand (Operand i64 Operand) Operand)
(function SubstBody (Body i64 Operand) Body)
(function SubstVecOperand (VecOperand i64 Operand) VecOperand)
(function SubstVecVecOperand (VecVecOperand i64 Operand) VecVecOperand)

Updated the egglog revision we depend on as this feature relies on vec-set: egraphs-good/egglog#239

Testing

Added tests to src/rvsdg/tests.rs

[oflatt]

Requested some changes, this is very close to what we want!

[oflatt]

Can you add a ruleset for the subst rules to make it more usable?
I'd also like the existing egglog encoding to run this ruleset to saturation in-between runs of the normal ruleset.

[rtjoa]

Added all rules to the subst ruleset, added the existing rules to constant-fold +/* to the cfold ruleset, and made the schedule the following:

let schedule = format!("(run-schedule (repeat {} cfold (saturate subst)))", self.num_iters);```

[oflatt]

good job!