Rework fold_constants so it is recursion_safe

[aalexandrov]

Size estimate

S

Notes

Constant propagation through bindings is done in NormalizeLets, but the LetRec case currently returns an error.

LetRec implementation (basic)

Just remove the

Err(crate::TransformError::LetRecUnsupported)?;

clause from the MirRelationExpr::LetRec case in FoldConstants::action.

LetRec implementation (advanced)

We could to something similar to the "basic" or even of the "advanced" case of column_knowledge:

1. Introduce a context with the current bindings.
2. On LetRec, extend this context with the ids of the enclosing LetRec node and update those iteratively, descending sequentially in FoldConstants::action to each binding before descending to the body.
3. Doing the above step once is akin to how we estimate column_knowledge now, doing it until the bindings converge before we move to the body is akin to the proposed "advanced" scheme in column_knowledge.

Open questions

1. Can the inductive case from the Implementation sketch above diverge? (cc: @mgree I think in our offline discussion earlier today you came up with some good reasons against that concern)

[aalexandrov]

@ggevay assigning this to myself as this is one of the three that will also enable FuseAndCollapse (the others are redundant_join and projection_lifting).

[ggevay]

Can the inductive case from the Implementation sketch above diverge?

I think we'll have to limit the maximum number of iterations similarly to #16800