Optimize make-array for unknown dimensions.

(make-array x :element-type '(unsigned-byte 8)) went through a costly
procedure of determining what type should be used for
(unsigned-byte 8), but this can be done at compile-time.
That form is now 25 times faster, and only 4 times slower than
(make-array (the integer x) :element-type '(unsigned-byte 8))