objset: Give up and implement frozenset.

Tired of patching CPython stdlib for it.
diff --git a/py/mpconfig.h b/py/mpconfig.h
index f840414..561c328 100644
--- a/py/mpconfig.h
+++ b/py/mpconfig.h
@@ -235,6 +235,11 @@
 #define MICROPY_ENABLE_SLICE (1)
 #endif
 
+// Whether to support frozenset object
+#ifndef MICROPY_ENABLE_FROZENSET
+#define MICROPY_ENABLE_FROZENSET (1)
+#endif
+
 // Whether to support the property object
 #ifndef MICROPY_ENABLE_PROPERTY
 #define MICROPY_ENABLE_PROPERTY (1)