Rename rt_* to mp_*.

Mostly just a global search and replace.  Except rt_is_true which
becomes mp_obj_is_true.

Still would like to tidy up some of the names, but this will do for now.
diff --git a/py/builtinevex.c b/py/builtinevex.c
index d658db0..92550a7 100644
--- a/py/builtinevex.c
+++ b/py/builtinevex.c
@@ -43,7 +43,7 @@
     }
 
     // complied successfully, execute it
-    return rt_call_function_0(module_fun);
+    return mp_call_function_0(module_fun);
 }
 
 STATIC mp_obj_t mp_builtin_eval(mp_obj_t o_in) {
@@ -55,8 +55,8 @@
 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 = rt_globals_get();
-    mp_map_t *old_locals = rt_locals_get();
+    mp_map_t *old_globals = mp_globals_get();
+    mp_map_t *old_locals = mp_locals_get();
     if (n_args > 1) {
         mp_obj_t globals = args[1];
         mp_obj_t locals;
@@ -65,13 +65,13 @@
         } else {
             locals = globals;
         }
-        rt_globals_set(mp_obj_dict_get_map(globals));
-        rt_locals_set(mp_obj_dict_get_map(locals));
+        mp_globals_set(mp_obj_dict_get_map(globals));
+        mp_locals_set(mp_obj_dict_get_map(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
-    rt_globals_set(old_globals);
-    rt_locals_set(old_locals);
+    mp_globals_set(old_globals);
+    mp_locals_set(old_locals);
     return res;
 }