ChaCha20: Use ArraySplitMap to construct Iv.

This is a step towards eliminating the `unsafe` in `chunks_fixed()`.

src/aead/chacha.rs

@@ -193,8 +193,7 @@ pub struct Iv([u32; 4]);
 
 impl Iv {
     fn assume_unique_for_key(value: [u8; 16]) -> Self {
-        let value: &[[u8; 4]; 4] = value.chunks_fixed();
-        Self(value.map(u32::from_le_bytes))
+        Self(value.array_split_map(u32::from_le_bytes))
     }
 
     fn into_counter_for_single_block_less_safe(self) -> Counter {

src/polyfill/array_split_map.rs

@@ -27,3 +27,16 @@ impl<I, O> ArraySplitMap<I, O, 4, 3> for [I; 12] {
         ]
     }
 }
+
+impl<I, O> ArraySplitMap<I, O, 4, 4> for [I; 16] {
+    #[inline]
+    fn array_split_map(self, f: impl Fn([I; 4]) -> O) -> [O; 4] {
+        let [a0, a1, a2, a3, b0, b1, b2, b3, c0, c1, c2, c3, d0, d1, d2, d3] = self;
+        [
+            f([a0, a1, a2, a3]),
+            f([b0, b1, b2, b3]),
+            f([c0, c1, c2, c3]),
+            f([d0, d1, d2, d3]),
+        ]
+    }
+}

src/polyfill/chunks_fixed.rs

@@ -27,7 +27,6 @@ macro_rules! define_chunks_fixed {
 }
 
 // Sorted by the first value, then the second value.
-define_chunks_fixed!(16, 4);
 define_chunks_fixed!(16, 8);
 define_chunks_fixed!(32, 4);
 define_chunks_fixed!(64, 32);