bug#55414: 29.0.50; Byte compilation error for the modus-themes

[Mattias Engdegård]

> The patch below should significantly reduce the recursion depth (and
> hopefully make it faster, to boot) because the (loop (cdr form)) call is
> in tail position (and named-let should hence apply TCO to it).

Sorry Stefan, I didn't see your message until after pushing my change. If you 
prefer yours, feel free to swap. (They should be equivalent.)