Convert while loops into for loops

I used a very powerful tool [coccinelle](https://gitlab.inria.fr/coccinelle/coccinelleforrust) (its C version is used for Linux kernel refactorings).  Highly recommend installing it as it has a potential to simplify refactorings while not dealing with regex errors.

To install it, run

```sh
cargo install --git https://gitlab.inria.fr/coccinelle/coccinelleforrust
```

Afterward, create this file as `target/repl2.cocci` with the following content. Using `target/` ensures that it will be ignored by git.

```diff
@@
expression i, end;
@@

-i = 0usize;
-while i < end
+for i in 0usize..end
-{
   {
   ...
   }
-  i = i.wrapping_add(1);
-}
```

and run this command from the root of the repo:

```shell
cfr -c target/repl2.cocci src/ --apply
cargo fmt --all
```

Afterwards, I compiled and removed all unused index variables.

src/enc/bit_cost.rs

@@ -291,28 +291,20 @@ pub fn BrotliPopulationCost<HistogramType: SliceWrapper<u32> + CostAccessors>(
     if count == 4i32 {
         let mut histo: [u32; 4] = [0; 4];
 
-        i = 0usize;
-        while i < 4usize {
-            {
-                histo[i] = histogram.slice()[s[i]];
-            }
-            i = i.wrapping_add(1);
+        for i in 0usize..4usize {
+            histo[i] = histogram.slice()[s[i]];
         }
-        i = 0usize;
-        while i < 4usize {
-            {
-                let mut j: usize;
-                j = i.wrapping_add(1);
-                while j < 4usize {
-                    {
-                        if histo[j] > histo[i] {
-                            histo.swap(j, i);
-                        }
+        for i in 0usize..4usize {
+            let mut j: usize;
+            j = i.wrapping_add(1);
+            while j < 4usize {
+                {
+                    if histo[j] > histo[i] {
+                        histo.swap(j, i);
                     }
-                    j = j.wrapping_add(1);
                 }
+                j = j.wrapping_add(1);
             }
-            i = i.wrapping_add(1);
         }
         let h23: u32 = histo[2].wrapping_add(histo[3]);
         let histomax: u32 = brotli_max_uint32_t(h23, histo[0]);