Add an Affinity::display() method.

Make it possible to display affinities given a RegInfo reference.

lib/cretonne/src/isa/registers.rs

@@ -153,6 +153,12 @@ impl RegClassData {
     }
 }
 
+impl fmt::Display for RegClassData {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.write_str(self.name)
+    }
+}
+
 /// A small reference to a register class.
 ///
 /// Use this when storing register classes in compact data structures. The `RegInfo::rc()` method
@@ -176,6 +182,12 @@ impl From<RegClass> for RegClassIndex {
     }
 }
 
+impl fmt::Display for RegClassIndex {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "rci{}", self.0)
+    }
+}
+
 /// Information about the registers in an ISA.
 ///
 /// The `RegUnit` data structure collects all relevant static information about the registers in an