Update minesweeper.rs

Using as_bytes instead of chars() since All the inputs and outputs are in ASCII. Rust Strings and &str are utf8, so while one might expect "Hello".chars() to be simple, it has to check each char to see if it's 1, 2, 3, or 4 u8s long. If we know a &str is ASCII then we can call .as_bytes() and refer to the underlying data as a &[u8] (byte slice). Iterating over a slice of ASCII bytes is much quicker as there are no codepoints involved - every ASCII byte is one u8 long.

exercises/practice/minesweeper/tests/minesweeper.rs

@@ -5,7 +5,8 @@ fn remove_annotations(board: &[&str]) -> Vec<String> {
 }
 
 fn remove_annotations_in_row(row: &str) -> String {
-    row.chars()
+    row.as_bytes()
+        .iter()
         .map(|ch| match ch {
             '*' => '*',
             _ => ' ',