py: Make globals and locals proper dictionary objects.

Finishes addressing issue #424.

In the end this was a very neat refactor that now makes things a lot
more consistent across the py code base.  It allowed some
simplifications in certain places, now that everything is a dict object.

Also converted builtins tables to dictionaries.  This will be useful
when we need to turn builtins into a proper module.
diff --git a/py/builtinevex.c b/py/builtinevex.c
index 4aafbdb..0a92819 100644
--- a/py/builtinevex.c
+++ b/py/builtinevex.c
@@ -54,8 +54,8 @@
 STATIC mp_obj_t mp_builtin_exec(uint n_args, const mp_obj_t *args) {
     // Unconditional getting/setting assumes that these operations
     // are cheap, which is the case when this comment was written.
-    mp_map_t *old_globals = mp_globals_get();
-    mp_map_t *old_locals = mp_locals_get();
+    mp_obj_dict_t *old_globals = mp_globals_get();
+    mp_obj_dict_t *old_locals = mp_locals_get();
     if (n_args > 1) {
         mp_obj_t globals = args[1];
         mp_obj_t locals;
@@ -64,8 +64,8 @@
         } else {
             locals = globals;
         }
-        mp_globals_set(mp_obj_dict_get_map(globals));
-        mp_locals_set(mp_obj_dict_get_map(locals));
+        mp_globals_set(globals);
+        mp_locals_set(locals);
     }
     mp_obj_t res = parse_compile_execute(args[0], MP_PARSE_FILE_INPUT);
     // TODO if the above call throws an exception, then we never get to reset the globals/locals