[compiler] Improve handling of refs

[mvitousek]

Stack from ghstack (oldest at bottom):

• #30917
• -> #30902
• #30912

Summary: This change expands our handling of refs to build an understanding of nested refs within objects and functions that may return refs. It builds a special-purpose type system within the ref analysis that gives a very lightweight structural type to objects and array expressions (merging the types of all their members), and then propagating those types throughout the analysis (e.g., if ref has type Ref, then { x: ref } and [ref] have type Structural(value=Ref) and {x: ref}.anything and [ref][anything] have type Ref).

This allows us to support structures that contain refs, and functions that operate over them, being created and passed around during rendering without at runtime accessing a ref value.

The analysis here uses a fixpoint to allow types to be fully propagated through the system, and we defend against diverging by widening the type of a variable if it could grow infinitely: so, in something like

let x = ref;
while (condition) {
  x = [x]
}

we end up giving x the type Structural(value=Ref).