Hello,

The attached features work like `cl-maplist` and `cl-mapl`, applying 
functions to a sequence and to the remaining parts of a sequence.  For 
example, `(seq-mapsub #'identity [1 2 3])` returns `([1 2 3] [2 3] [3])`.

The patch adds a `seq-mapsub`, `seq-dosub`, and a `seq-doseqsub`, 
similar to `seq-map`, `seq-do`, and `seq-doseq`, respectively.

I was looking for an equivalent for vectors of `cl-maplist`, `cl-mapl`, 
and `cl-loop`'s `for VAR on LIST`, and think that these would be useful 
additions.

To get the sub-sequences, the code uses `seq-rest` and stops when the 
returned sub-sequence is empty according to `seq-empty-p`.  This is 
similar to how I would do it for a list, using `cdr` and `null`, but is 
that a good way to do it for arrays and other sequences?

Thank you.