PR #2059 fixes the wrong handling of LoadFunction, but now extension constant folders will not see the LoadedFunctions (previously they would have seen Value::Functions, albeit malformed). This means we'll need a new enum, replacing use of Value, to pass into https://github.com/CQCL/hugr/blob/ff0252e4d7d691faea47fa5c4ca4a1f73ead2b67/hugr-core/src/extension/const_fold.rs#L33