Fix: computedFn should call onCleanup when computation is not memoized

[wemyss]

Currently when you invoke computedFn outside of a reactive context, the provided onCleanup method is never called. This differs from createTransformer -

mobx-utils/src/create-transformer.ts

Line 113 in f4d31e8

if (onCleanup) onCleanup(value, object)