py: Add support for __debug__ constant.

__debug__ now resolves to True or False.  Its value needs to be set by
mp_set_debug().

TODO: call mp_set_debug in unix/ port.

TODO: optimise away "if False:" statements in compiler.
diff --git a/py/runtime.c b/py/runtime.c
index 8852b7d..3cdbd22 100644
--- a/py/runtime.c
+++ b/py/runtime.c
@@ -69,11 +69,14 @@
 };
 
 void mp_init(void) {
-    // call port specific initialization if any 
+    // call port specific initialization if any
 #ifdef MICROPY_PORT_INIT_FUNC
     MICROPY_PORT_INIT_FUNC;
 #endif
 
+    // __debug__ enabled by default
+    mp_set_debug(true);
+
     mp_emit_glue_init();
 
     // init global module stuff