Refactored contracts (#2960)

* - Properly support `@deprecated` as contract.
- Support deprecating enum values.
- Refactor contracts

* Always copy enum constants.

* Bugfix.

test/test_suite/contracts/contract_deprecation.c3

@@ -0,0 +1,15 @@
+constdef Foo
+{
+	<* @deprecated *>
+	ABC
+}
+
+<* @deprecated *>
+Foo a;
+fn int main()
+{
+
+	Foo f = Foo.ABC; // #warning: 'ABC' is deprecated.
+	Foo b = a;       // #warning: 'a' is deprecated.
+	return 0;
+}