Module Incremental_kernel__.Should_not_use

A module internal to Incremental. Users should see Incremental_intf.

In several places, we use Should_not_use.t to implement a "lightweight" existential. I.e. for some type 'a t, we "pack" it by using Obj.magic to cast to Should_not_use.t t. This is OK so long as we we never mix the Should_not_use.t parts from two different types that have been so packed.

We do this for performance reasons, to avoid the boxing and indirection that comes from OCaml's implementation of packing. It can also make pointer-manipulation code more transparent.