char: Mark the MAX constant stable

Author: brson

src/libcore/char.rs

@@ -63,6 +63,7 @@ static MAX_THREE_B: u32 =  0x10000u32;
 */
 
 /// The highest valid code point
+#[stable]
 pub const MAX: char = '\U0010ffff';
 
 /// Converts from `u32` to a `char`