Allow seq_expr, not only expr as argument of attributes/extension nodes.  This allows us to write [%do a; b; c <-- d; f c; ...].

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/extension_points@13376 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
Support attributes on opening delimiter/keyword for all kinds of expressions.

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/extension_points@13377 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02