Hello,

This patch adds support for using `seq-subseq` with `setf`, as in

     ;; => [0 1 2 10 11]
     (let ((seq (vector 0 1 2 3 4)))
       (setf (seq-subseq seq -2) (list 10 11 12 13 14))
       seq)

The patch adds a generic version which uses the existing `setf` support 
of `seq-elt` and a specialized version for modifying lists.  Both 
versions use `seq-do` to map a function over the values that should 
replace the values in the modified sequence.

To avoid modifying more values than specified, that modifying function 
uses a `when` condition. I'm not sure of a good way to stop `seq-do` 
early when we know that it can stop calling the modifying function. 
Normally, I would use `cl-block` and `cl-return`. Is it OK to use those 
features in `seq.el`? If not, is it worth adding something like a 
`seq-map-while` or a `seq-do-while`?

Thank you.