Nonconformant simple vs extended loop detection

According to 6.1.1.1, a loop is a simple loop if it only contains compound forms. Currently only the first form is considered via

  (symbolp (car keywords-and-forms))

in `loop-standard-expansion'.

Under a conformant implementation the form

  (loop (foo) for)

must signal an error.

The fix appears to be to replace the condition in `loop-standard-expansion' with

  (some 'atom keywords-and-forms)