bug#68863: Add support for using setf with seq-subseq

[Stefan Monnier]

>> 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.
> Nicolas, Stefan: any comments?

Fine by me.

>> 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`?

`seq.el` is used by some parts of the implementation of `cl-lib`, so
the use of `cl-lib` risks introducing a circular dependency.  Maybe using
`cl-block/return` would be OK, but I wouldn't be surprised if it causes
bootstrap trouble.  You can use catch/throw, OTOH.


        Stefan