Add source file name and line number to error messages.

Byte code has a map from byte-code offset to source-code line number,
used to give better error messages.
diff --git a/py/parse.h b/py/parse.h
index be2073a..e1e7f73 100644
--- a/py/parse.h
+++ b/py/parse.h
@@ -28,7 +28,7 @@
 typedef machine_uint_t mp_parse_node_t; // must be pointer size
 
 typedef struct _mp_parse_node_struct_t {
-    uint32_t source;            // file identifier, and line number
+    uint32_t source_line;       // line number in source file
     uint32_t kind_num_nodes;    // parse node kind, and number of nodes
     mp_parse_node_t nodes[];    // nodes
 } mp_parse_node_struct_t;