Generate an opcode_name() function.

This function returning a &'static str is more primitive that the Display
implementation. It allows the opcode strings to be reused by the parser.

src/libcretonne/immediates.rs

@@ -8,9 +8,16 @@
 use std::fmt::{self, Display, Formatter};
 use std::mem;
 
-// The `Opcode` enum is generated from the meta instruction descriptions.
+// The `Opcode` enum and the `opcode_name` function are generated from the meta instruction
+// descriptions.
 include!(concat!(env!("OUT_DIR"), "/opcodes.rs"));
 
+impl Display for Opcode {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        write!(f, "{}", opcode_name(*self))
+    }
+}
+
 /// 64-bit immediate integer operand.
 ///
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]