Add some comments noting unimplemented features.

While use of these features will trigger an `unimplemented!()`, it is
nice to let users know in advance about features which aren't yet
implemented.

lib/cretonne/src/ir/heap.rs

@@ -25,6 +25,8 @@ pub struct HeapData {
 #[derive(Clone)]
 pub enum HeapBase {
     /// The heap base lives in a reserved register.
+    ///
+    /// This feature is not yet implemented.
     ReservedReg,
 
     /// The heap base is in a global variable.

lib/cretonne/src/isa/stack.rs

@@ -68,6 +68,8 @@ pub enum StackBase {
     FP = 1,
 
     /// Use an explicit zone pointer in a general-purpose register.
+    ///
+    /// This feature is not yet implemented.
     Zone = 2,
 }