Fix formatting of control characters \x1a through \x1f

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

Fix the range, and add a test.

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\""
```

src/impls.rs

@@ -539,7 +539,7 @@ mod bstr {
                     '\x01'..='\x08'
                     | '\x0b'
                     | '\x0c'
-                    | '\x0e'..='\x19'
+                    | '\x0e'..='\x1f'
                     | '\x7f' => {
                         write!(f, "\\x{:02x}", ch as u32)?;
                     }
@@ -1305,7 +1305,12 @@ 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