Expose result of LoadFunction into constant folding

[acl-cqc]

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

hugr/hugr-core/src/extension/const_fold.rs

Line 33 in ff0252e

fn fold(