hanging package makes ' unusable in math


```
\documentclass{article}
\usepackage{hanging}
\begin{document}
\[K' x\]
\end{document}
```

Produces

```
! TeX capacity exceeded, sorry [input stack size=10000].
'->\futurelet 
              \next \h@ngrqtest 
l.4 \[K' 
         x\]
!  ==> Fatal error occurred, no output PDF file produced!
```

As the non-active `'` is re-inserted, but as this is math-active, it loops.
