Make the split flag obsolete

[okamsn]

What the split flag does is good, but is a more restricted version of what
accum-opt can do.

;; => ((1 2 3) (3 6 9))
(loopy (flag split)
       (nums i 1 3)
       (collect i)
       (collect (* 3 i)))

;; => ((1 2 3) (3 6 9))
(loopy (accum-opt coll1 coll2)
       (nums i 1 3)
       (collect coll1 i)
       (collect coll2 (* 3 i))
       (finally-return coll1 coll2))

The split flag cannot be used outside of a one-to-one correspondence of
accumulation variables and commands. Since accum-opt has explicit variable
names, it does not have this restriction.

;; `split' cannot be used like this:
;; => ((1 2 3) (3 4 6 8 9 12))
(loopy (accum-opt coll1 coll2)
       (nums i 1 3)
       (collect coll1 i)
       (collect coll2 (* 3 i))
       (collect coll2 (* 4 i))
       (finally-return coll1 coll2))

Since the accump-opt special macro argument is more general and more clear,
split should be obsoleted and eventually removed.

[okamsn]

Done in PR #131.