vim: return old-style completion list when v:version <= 703

It seems that v:version is not enough to discover expected format for
completion function results. Use old-style by default now.
Fix grammar: support top-level expressions

Ocaml grammar allows both seq_expr and structure_item to occur in a
structure, merlin dropped support for seq_expr.
This should be fixed now.
RFC: allow recursive loading of .merlin file