Function canonicalize

Canonicalizes value into a deterministic string, or undefined if it cannot be safely canonicalized (a function, a symbol, a cyclic reference, an invalid Date, a class instance without .toJSON(), or any object carrying __proto__/constructor/prototype as an own key). Never throws. Equal inputs always produce equal output; distinct supported values never collide (own-enumerable-key walk only, Object.keys -- never for...in).