Hide grn_snip struct

It is a backward incompatible change.

Here is migration guide:

  * Rename grn_snip to grn_obj:

    Before:

        grn_snip *snip;

    After:

        grn_obj *snip;

  * Use grn_obj_close() instead of grn_snip_close():

    Before:

        grn_snip_close(ctx, snip);

    After:

        grn_obj_close(ctx, snip);