Revert "Change to non-native type"

This reverts commit 651c475.

Alas, the PR does not seem to be ready yet :-(

src/core/Enumeration.pm6

@@ -2,7 +2,7 @@
 my role Enumeration {
     has $.key;
     has $.value;
-    has Int $!index;
+    has int $!index;
 
     method enums() { self.^enum_values.Map }