py: Add module/function/class name to exceptions.

Exceptions know source file, line and block name.

Also tidy up some debug printing functions and provide a global
flag to enable/disable them.
diff --git a/py/parse.h b/py/parse.h
index e1e7f73..2801f41 100644
--- a/py/parse.h
+++ b/py/parse.h
@@ -54,7 +54,7 @@
 
 mp_parse_node_t mp_parse_node_new_leaf(machine_int_t kind, machine_int_t arg);
 
-void mp_parse_node_show(mp_parse_node_t pn, int indent);
+void mp_parse_node_print(mp_parse_node_t pn, int indent);
 
 typedef enum {
     MP_PARSE_SINGLE_INPUT,