Split seq.el into separate files for different versions of Emacs

All functions in seq-25.el are wrapped in a `seq--when-emacs-25-p' to
make sure that the byte compiler won't emit warnings or errors when the
file is byte compiled in Emacs < 25.

* packages/seq/seq-24.el:
* packages/seq/seq-25.el: New files.
* packages/seq/seq.el: Load seq-VERSION.el based on the version of Emacs.
* packages/seq/test/seq.el-test.el: Backport a test from seq.el in Emacs
  master.