Support interface composition (<+>) and abstraction (Use) in FreeSpec.Exec

[Thomas Letan]

Currently, the examples of FreeSpec.Exec only works because the typeclass inference of Coq eagerly replaces {Use i ix}, ix into i.

Using two different interfaces breaks that trick, as shown by the following example:

Require Import Prelude.Control.
Require Import FreeSpec.Program.

Inductive i1 (a: Type): Type := wrap: a -> i1 a.
Inductive i2 (a: Type): Type := unwrap: i2 a.

Arguments wrap [a] (_).
Arguments unwrap [a].

Local Open Scope prelude_scope.

Definition test {ix} `{Use i1 ix} `{Use i2 ix} : Program ix nat :=
  request (wrap 2) ;;
  request unwrap.

Eval compute in test.

Output is:

     = Request ((let (lift_eff) := ?H in lift_eff) nat (wrap 2))
         (fun _ : nat =>
          Request ((let (lift_eff) := ?H0 in lift_eff) nat unwrap) (Pure (A:=nat)))
     : Program ?ix nat

Similarly, using the <+> operator to compose interfaces together would also break FreeSpec.Exec.