emacs: merlin-type-expression-node.
emacs: improved ocamlatom definition.
emacs: factorized out the management of temporary overlays.
emacs: fixed a bug in create-overlay
emacs: merlin-point tell to the next phrase

Emacs mode now uses the tell end feature. Using merlin-point now synchronizes to
the end of the phrase that contains the point.
emacs: type: trying the node approximation first.
emacs: simplified management of type-overlay