Simplify [Dep_node] (#2986)
Simplify [Dep_node] (#2986)

We use a combination of [mutable] and [Lazy.t] to initialise [dep_node] with a
reference to a [dag_node] that needs to refer back to the [dep_node], in a cyclic
manner. It turns out we can achieve this simply by using a recursive let binding.

Signed-off-by: Andrey Mokhov <251bbb147316c69f1038239e09718efe08bbf2d1@janestreet.com>
janestreet/jbuilder,dra27/jbuilder,dra27/jbuilder,janestreet/jbuilder,dra27/jbuilder,dra27/jbuilder,janestreet/jbuilder,dra27/jbuilder,janestreet/jbuilder,janestreet/jbuilder,dra27/jbuilder,janestreet/jbuilder