Add an `insn_array` array_class to `comedi.i`.

Add an `insn_array` array_class to `comedi.i`.

This makes it possible to populate `comedi_insnslist_struct` for use
with `comedi_do_insnlist`.

I also removed my earlier re-typedefs of `sampl_array` and
`lsampl_array`.  I didn't understand the reason for the underlying
`TypeError` when I added them.  The proper way to avoid that error is
to use the `.cast()` and `.frompointer()` methods.  See
`demo/python/insn.py` (coming soon) for details.