Python builtin hashfunc closure fix

If the wrong return type in the hash function was used, an error:
  SystemError: error return without exception set
was raised

Add some tests for testing builtin slots

Examples/test-suite/python_builtin.i

@@ -0,0 +1,74 @@
+// Test customizing slots when using the -builtin option
+
+%module python_builtin
+
+%inline %{
+#ifdef SWIGPYTHON_BUILTIN
+bool is_python_builtin() { return true; }
+#else
+bool is_python_builtin() { return false; }
+#endif
+%}
+
+// Test 1 for tp_hash
+#if defined(SWIGPYTHON_BUILTIN)
+%feature("python:tp_hash") SimpleValue "SimpleValueHashFunction"
+#endif
+
+%inline %{
+struct SimpleValue {
+  int value;
+  SimpleValue(int value) : value(value) {}
+  static SimpleValue *inout(SimpleValue *sv) {
+    return sv;
+  }
+};
+%}
+
+%{
+#if PY_VERSION_HEX >= 0x03020000
+Py_hash_t SimpleValueHashFunction(PyObject *v)
+#else
+long SimpleValueHashFunction(PyObject *v)
+#endif
+{
+  SwigPyObject *sobj = (SwigPyObject *) v;
+  SimpleValue *p = (SimpleValue *)sobj->ptr;
+  return p->value;
+}
+hashfunc test_hashfunc_cast() {
+    return SimpleValueHashFunction;
+}
+%}
+
+// Test 2 for tp_hash
+#if defined(SWIGPYTHON_BUILTIN)
+%feature("python:slot", "tp_hash", functype="hashfunc") BadHashFunctionReturnType::bad_hash_function;
+#endif
+
+%inline %{
+struct BadHashFunctionReturnType {
+    static const char * bad_hash_function() {
+      return "bad hash function";
+    }
+};
+%}
+
+// Test 3 for tp_hash
+#if defined(SWIGPYTHON_BUILTIN)
+%feature("python:slot", "tp_hash", functype="hashfunc") ExceptionHashFunction::exception_hash_function;
+#endif
+
+%catches(const char *) exception_hash_function;
+
+%inline %{
+#if PY_VERSION_HEX < 0x03020000
+  #define Py_hash_t long
+#endif
+struct ExceptionHashFunction {
+    static Py_hash_t exception_hash_function() {
+      throw "oops";
+    }
+};
+%}
+