A module internal to Incremental. Users should see Incremental_intf.

An on-update handler is stored in a node or an observer, and is run at the end of a stabilization either when the value of the node changes, when the handler is installed, or when the node becomes invalid.

`module Node_update : sig ... end`
`type 'a t`````
`include sig ... end`
`val sexp_of_t : ('a ‑> Base.Sexp.t) ‑> 'a t ‑> Base.Sexp.t`
`val create : ('a Node_update.t ‑> unit) ‑> at:Incremental_kernel__.Stabilization_num.t ‑> 'a t`
`val run : 'a t ‑> 'a Node_update.t ‑> now:Incremental_kernel__.Stabilization_num.t ‑> unit`