feat: add a Risc0 backend

Defaults to software keccak on unoptimized rounds

keccak/src/backends/risc0.rs

@@ -0,0 +1,23 @@
+use crate::consts::F1600_ROUNDS;
+use crate::types::{Fn1600, State1600};
+#[cfg(feature = "parallel")]
+use hybrid_array::typenum::U1;
+
+pub(crate) struct Backend;
+
+impl super::Backend for Backend {
+    #[cfg(feature = "parallel")]
+    type ParSize1600 = U1;
+
+    #[inline]
+    fn get_p1600<const ROUNDS: usize>() -> Fn1600 {
+        if ROUNDS == F1600_ROUNDS {
+            return risc0_p1600;
+        }
+        super::soft::keccak_p::<u64, ROUNDS>
+    }
+}
+
+fn risc0_p1600(state: &mut State1600) {
+    risc0_zkvm::guest::env::risc0_keccak_update(state);
+}