Add Poison.encode_to_iodata(!)/2

Having an explicit function plays nicer with dialyzer
and makes Poison simpler to use as callbacks.

Ideally we would deprecate the :iodata option in
encode and titghen its typespecs but I haven't done
so in this commit in order to keep backwards
compatibility.

Pleeeeeease merge this and release 1.3. It would help a lot
in Phoenix too.