core: ensure that there no multiple occupancies of classes produced by makeResetStyles in mergeClasses calls

[layershifter]

See #225 for details.

We should add a runtime validation to mergeClasses() to ensure that the following scenario is impossible:

const useClassNameA = makeResetStyles()
const useClassNameB = makeResetStyles()

function App() {
  const classNameA = useClassNameA()
  const classNameB = useClassNameB()

  return <div className={mergeClasses(classNameA, classNameB)} />
}