Improve seq-concatenate for new sequence types

Improve seq-concatenate for new sequence types

Use the new `seq-into-sequence' in seqs passed to `seq-concatenate' to
ensure that concatenation happens on sequences only.  This makes it
possible to use `seq-concatenate' for new types of seqs.

* lisp/emacs-lisp/seq.el (seq-into-sequence, seq-concatenate): New
function used in `seq-concatenate'.
* test/automated/seq-tests.el (test-seq-into-sequence): New unit test
for seq-into-sequence.