impl: fix formatting of control characters \x1a through \x1f in Debug…

… impl

Due to an incorrect range, control characters \x1a through \x1f get
formatted as Unicode \u escapes rather than hex \x escapes.

This commit fixes the range, adds a test and switches over to std's
`escape_ascii` to do more of the formatting work.

Without the fix, the test fails like this:

```
  left: "\"\\0\\x01\\x02\\x03\\x04\\x05\\x06\\x07\\x08\\t\\n\\x11\\x12\\r\\x14\\x15\\x16\\x17\\x18\\x19\\x1a\\x1b\\x1c\\x1d\\x1e\\x1f \\x7f\\x80\\x81\\xfe\\xff\""
 right: "\"\\0\\x01\\x02\\x03\\x04\\x05\\x06\\x07\\x08\\t\\n\\x11\\x12\\r\\x14\\x15\\x16\\x17\\x18\\x19\\u{1a}\\u{1b}\\u{1c}\\u{1d}\\u{1e}\\u{1f} \\x7f\\x80\\x81\\xfe\\xff\""
```

PR #201

src/impls.rs

@@ -525,6 +525,9 @@ mod bstr {
             for (s, e, ch) in self.char_indices() {
                 match ch {
                     '\0' => write!(f, "\\0")?,
+                    '\x01'..='\x7f' => {
+                        write!(f, "{}", (ch as u8).escape_ascii())?;
+                    }
                     '\u{FFFD}' => {
                         let bytes = self[s..e].as_bytes();
                         if bytes == b"\xEF\xBF\xBD" {
@@ -535,17 +538,6 @@ mod bstr {
                             }
                         }
                     }
-                    // ASCII control characters except \0, \n, \r, \t
-                    '\x01'..='\x08'
-                    | '\x0b'
-                    | '\x0c'
-                    | '\x0e'..='\x19'
-                    | '\x7f' => {
-                        write!(f, "\\x{:02x}", ch as u32)?;
-                    }
-                    '\n' | '\r' | '\t' => {
-                        write!(f, "{}", ch.escape_debug())?;
-                    }
                     _ => {
                         write!(f, "{}", ch.escape_debug())?;
                     }
@@ -1305,7 +1297,12 @@ fn test_debug() {
         // Before fixing #188, the output here would be:
         //   \\xED\\xA0\\x80Aa\\x7f\\x0b
         B(&format!("{:?}", b"\xed\xa0\x80Aa\x7f\x0b".as_bstr())).as_bstr(),
-    )
+    );
+
+    assert_eq!(
+        r#""\0\x01\x02\x03\x04\x05\x06\x07\x08\t\n\x11\x12\r\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f \x7f\x80\x81\xfe\xff""#,
+        format!("{:?}", b"\0\x01\x02\x03\x04\x05\x06\x07\x08\t\n\x11\x12\r\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f \x7f\x80\x81\xfe\xff".as_bstr()),
+    );
 }
 
 // See: https://github.com/BurntSushi/bstr/issues/82