ascii::Char-ify the escaping code

This means that `EscapeIterInner::as_str` no longer needs unsafe code, because the type system ensures the internal buffer is only ASCII, and thus valid UTF-8.

library/core/src/ascii.rs

@@ -91,7 +91,7 @@ pub struct EscapeDefault(escape::EscapeIterInner<4>);
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub fn escape_default(c: u8) -> EscapeDefault {
-    let mut data = [0; 4];
+    let mut data = [Char::Null; 4];
     let range = escape::escape_ascii_into(&mut data, c);
     EscapeDefault(escape::EscapeIterInner::new(data, range))
 }