Implement Iterator trait for Fragment and BoundedVec (#327)

This PR creates Iterator trait with one function: `for_each` and
implements it for Fragment and BoundedVec.
It also migrates the code to use `for_each` where possible. I only found
2 places (1 for Fragment and 1 for BoundedVec) where it's possible to
use it. In the rest of the loops iterating over fragments or boundedvecs
`i` (loop iterator) is used in some way that makes it impossible to use
`for_each`.

ethereum/circuits/lib/src/misc.nr

@@ -11,3 +11,4 @@ mod types;
 mod option;
 mod bytes32;
 mod bytes32_test;
+mod iterator;

ethereum/circuits/lib/src/misc/bounded_vecs.nr

@@ -1,3 +1,5 @@
+use crate::misc::iterator::Iterator;
+
 pub fn bounded_vec_from_array<T, N, M>(array: [T; N]) -> BoundedVec<T, M> {
     assert(N <= M, "Array size exceeds bounded vector size");
     let mut vec = BoundedVec::new();
@@ -10,11 +12,18 @@ pub fn bounded_vec_map<U, T, N, Env>(
     bounded_vec: BoundedVec<T, N> ,
     f: fn[Env](T) -> U
 ) -> BoundedVec<U, N> {
-    let mut result: BoundedVec<U, N> = BoundedVec::new();
-    for i in 0..N {
-        if (i < bounded_vec.len) {
-            result.push(f(bounded_vec.get(i)));
+    let result: &mut BoundedVec<U, N> = &mut BoundedVec::new();
+    bounded_vec.for_each(|x| result.push(f(x)));
+
+    *result
+}
+
+impl<T, N> Iterator<T> for BoundedVec<T, N> {
+    fn for_each<Env>(self, f: fn[Env](T) -> ()) {
+        for i in 0..N {
+            if i < self.len {
+                f(self.get(i));
+            }
         }
     }
-    result
 }

ethereum/circuits/lib/src/misc/fragment.nr

@@ -1,4 +1,4 @@
-use crate::misc::arrays::{memcpy_up_to_length, resize};
+use crate::misc::{arrays::{memcpy_up_to_length, resize}, iterator::Iterator};
 use dep::std::unsafe::zeroed;
 
 struct Fragment<MAX_DATA_LEN, T> {
@@ -38,13 +38,10 @@ impl<MAX_DATA_LEN, T> Fragment<MAX_DATA_LEN, T> {
 
     pub fn to_bounded_vec<N>(self) -> BoundedVec<T, N> {
         assert(self.length <= N, "Fragment length exceeds BoundedVec max length");
-        let mut bounded_vec: BoundedVec<T, N> = BoundedVec::new();
-        for i in 0..N {
-            if (i < self.length) {
-                bounded_vec.push(self.at(i));
-            }
-        }
-        bounded_vec
+        let bounded_vec: &mut BoundedVec<T, N> = &mut BoundedVec::new();
+        self.for_each(|x| bounded_vec.push(x));
+
+        *bounded_vec
     }
 
     pub fn from_vec(vec: BoundedVec<T, MAX_DATA_LEN>) -> Fragment<MAX_DATA_LEN, T> {
@@ -150,3 +147,13 @@ impl<MAX_DATA_LEN, T> Eq for Fragment<MAX_DATA_LEN, T> where T: Eq {
         res
     }
 }
+
+impl<MAX_DATA_LEN, T> Iterator<T> for Fragment<MAX_DATA_LEN, T> {
+    fn for_each<Env>(self, f: fn[Env](T) -> ()) {
+        for i in 0..MAX_DATA_LEN {
+            if i < self.length {
+                f(self.at(i));
+            }
+        }
+    }
+}

ethereum/circuits/lib/src/misc/iterator.nr

@@ -0,0 +1,3 @@
+trait Iterator<T> {
+    fn for_each<Env>(self, f: fn[Env](T) -> ());
+}