Port FlattenSmartConstructors functor to Rust

Port FlattenSmartConstructors functor to Rust

Summary:
In OCaml, Flatten SmartConstructors (SC) is a SC specialization with:

    type r = Op.r

which is parameterized by `Op: Op_S`, where:

    module type Op_S = sig
      type r
      val is_zero: r -> bool
      val flatten: r list -> r
      val zero: r
    end

and provides a default implementation of `make_*` methods.  In Rust,
model this as a subtrait of SC with corresponding *default* methods that flatten:

    trait FlattenSmartConstructors<State>
    : SmartConstructors<State>
    + FlattenOp<S=<Self as SmartConstructors<State>>::R>

Reviewed By: dabek

Differential Revision: D14736865

fbshipit-source-id: 743d9c616aa088e1c731c98e1cf449d2728fd9b5