(ring-size, ring-p, ring-insert, ring-length, ring-empty-p): Use c[ad]dr.

(ring-size, ring-p, ring-insert, ring-length, ring-empty-p): Use c[ad]dr.
(ring-plus1): Use `1+'.
(ring-minus1): Use `zerop'.
(ring-remove): Use c[ad]dr.  Use `when'.
(ring-copy): Use c[ad]dr.  Use `let', not `let*'.
(ring-ref): Use `let', not `let*'.
(ring-insert-at-beginning): Use c[ad]dr.  Doc fix.
(ring-insert+extend): Use c[ad]dr.  Fix typo in docstring.
(ring-member): Simplify.  Doc fix.
(ring-convert-sequence-to-ring): Simplify.