4vecs-length
Get the length of the 4vecarr array.
In the execution this just gets the array length. The
logical definition is just a wrapper for len:
Function: 4vecs$a-length
(defun 4vecs$a-length (4vecarr$a)
(declare (xargs :guard (4vecarr$ap 4vecarr$a)))
(len 4vecarr$a))
