Add CryptographicSpongeExt to enable access to "gadget" version of …

…a `Sponge` (arkworks-rs#24)

* Add `CryptographicSpongeExt` to enable easy access to "gadget" version of a `Sponge`

* Update CHANGELOG

* Rename trait

* Update CHANGELOG

* Format

CHANGELOG.md

@@ -10,6 +10,8 @@
 
 - [\#22](https://github.com/arkworks-rs/sponge/pull/22) Add traits and derivations for default Poseidon parameters.
 
+- [\#24](https://github.com/arkworks-rs/sponge/pull/24) Add `SpongeWithGadget` trait that enables getting the gadget version of a sponge.
+
 ### Improvements
 
 ### Bug fixes

src/constraints/mod.rs

@@ -88,6 +88,13 @@ pub fn bits_le_to_nonnative<'a, F: PrimeField, CF: PrimeField>(
     Ok(output)
 }
 
+/// Enables simple access to the "gadget" version of the sponge.
+/// Simplifies trait bounds in downstream generic code.
+pub trait SpongeWithGadget<CF: PrimeField>: CryptographicSponge {
+    /// The gadget version of `Self`.
+    type Var: CryptographicSpongeVar<CF, Self>;
+}
+
 /// The interface for a cryptographic sponge constraints on field `CF`.
 /// A sponge can `absorb` or take in inputs and later `squeeze` or output bytes or field elements.
 /// The outputs are dependent on previous `absorb` and `squeeze` calls.

src/poseidon/constraints.rs

@@ -1,5 +1,5 @@
 use crate::constraints::AbsorbGadget;
-use crate::constraints::CryptographicSpongeVar;
+use crate::constraints::{CryptographicSpongeVar, SpongeWithGadget};
 use crate::poseidon::{PoseidonParameters, PoseidonSponge};
 use crate::DuplexSpongeMode;
 use ark_ff::{FpParameters, PrimeField};
@@ -30,6 +30,10 @@ pub struct PoseidonSpongeVar<F: PrimeField> {
     pub mode: DuplexSpongeMode,
 }
 
+impl<F: PrimeField> SpongeWithGadget<F> for PoseidonSponge<F> {
+    type Var = PoseidonSpongeVar<F>;
+}
+
 impl<F: PrimeField> PoseidonSpongeVar<F> {
     #[tracing::instrument(target = "r1cs", skip(self))]
     fn apply_s_box(