py: Implement support for "except Exception as var" clause.

For this, needed to implement DELETE_NAME bytecode (because var bound
in except clause is automatically deleted at its end).
http://docs.python.org/3/reference/compound_stmts.html#except :
"When an exception has been assigned using as target, it is cleared at
the end of the except clause."

py/runtime.c

@@ -462,6 +462,11 @@ void rt_store_name(qstr qstr, mp_obj_t obj) {
     mp_map_lookup(map_locals, MP_OBJ_NEW_QSTR(qstr), MP_MAP_LOOKUP_ADD_IF_NOT_FOUND)->value = obj;
 }
 
+void rt_delete_name(qstr qstr) {
+    DEBUG_OP_printf("delete name %s\n", qstr_str(qstr));
+    mp_map_lookup(map_locals, MP_OBJ_NEW_QSTR(qstr), MP_MAP_LOOKUP_REMOVE_IF_FOUND);
+}
+
 void rt_store_global(qstr qstr, mp_obj_t obj) {
     DEBUG_OP_printf("store global %s <- %p\n", qstr_str(qstr), obj);
     mp_map_lookup(map_globals, MP_OBJ_NEW_QSTR(qstr), MP_MAP_LOOKUP_ADD_IF_NOT_FOUND)->value = obj;

py/runtime.h

@@ -12,6 +12,7 @@ mp_obj_t rt_get_cell(mp_obj_t cell);
 void rt_set_cell(mp_obj_t cell, mp_obj_t val);
 void rt_store_name(qstr qstr, mp_obj_t obj);
 void rt_store_global(qstr qstr, mp_obj_t obj);
+void rt_delete_name(qstr qstr);
 mp_obj_t rt_unary_op(int op, mp_obj_t arg);
 mp_obj_t rt_binary_op(int op, mp_obj_t lhs, mp_obj_t rhs);
 mp_obj_t rt_make_function_from_id(int unique_code_id, mp_obj_t def_args);

py/vm.c

@@ -283,6 +283,11 @@ dispatch_loop:
                         sp -= 3;
                         break;
 
+                    case MP_BC_DELETE_NAME:
+                        DECODE_QSTR;
+                        rt_delete_name(qst);
+                        break;
+
                     case MP_BC_DUP_TOP:
                         obj1 = TOP();
                         PUSH(obj1);

tests/basics/try-as-var.py

@@ -0,0 +1,10 @@
+try:
+    raise ValueError(534)
+except ValueError as e:
+    print(repr(e))
+
+# Var bound in except block is automatically deleted
+try:
+    e
+except NameError:
+    print("NameError")