Fix grammar: support top-level expressions
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.

haselwarter/merlin,neojski/merlin,haselwarter/merlin,chambart/merlin,chriskonstad/merlin,fxfactorial/merlin,rgrinberg/merlin,dra27/merlin,haselwarter/merlin,chambart/merlin,dra27/merlin,the-lambda-church/merlin,rgrinberg/merlin,the-lambda-church/merlin,fxfactorial/merlin,fxfactorial/merlin,neojski/merlin,rgrinberg/merlin,haselwarter/merlin,ocaml/merlin,neojski/merlin,neojski/merlin,dra27/merlin,chambart/merlin,ocaml/merlin,chriskonstad/merlin,ocaml/merlin,fxfactorial/merlin,the-lambda-church/merlin,chambart/merlin,ocaml/merlin,dra27/merlin,rgrinberg/merlin