feat: ensure SMIR export records function item types

[Stevengre]

Problem

While exporting SMIR, stable-mir-json fails to record types for closure/function constants. Consumers that rely on lookupTy (e.g., KMIR) therefore fall back to typeInfoVoidType, which eventually produces thunk values and halts execution. This surfaced when running kmir prove-rs/kmir show on interior-mut-refcell.rs.

Changes

• Ensure TyCollector::visit_ty always inserts Closure, FnDef, and FnPtr types into self.types, even if visiting the instance short-circuits.
• In InternedValueCollector::visit_mir_const, always feed the constant’s own ty into the TyCollector, so zero-sized constants also get their metadata recorded.

Result

The generated *.smir.json now contains the missing function-item types. KMIR no longer falls back to typeInfoVoidType, and the previously stuck proof run completes without emitting thunk.