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/objbool.c b/py/objbool.c
index 5f17fd6..6afb6e9 100644
--- a/py/objbool.c
+++ b/py/objbool.c
@@ -26,7 +26,7 @@
 
     switch (n_args) {
         case 0: return mp_const_false;
-        case 1: if (rt_is_true(args[0])) { return mp_const_true; } else { return mp_const_false; }
+        case 1: if (mp_obj_is_true(args[0])) { return mp_const_true; } else { return mp_const_false; }
         default: nlr_jump(mp_obj_new_exception_msg_varg(&mp_type_TypeError, "bool takes at most 1 argument, %d given", n_args));
     }
 }
@@ -34,10 +34,10 @@
 STATIC mp_obj_t bool_unary_op(int op, mp_obj_t o_in) {
     machine_int_t value = ((mp_obj_bool_t*)o_in)->value;
     switch (op) {
-        case RT_UNARY_OP_BOOL: return o_in;
-        case RT_UNARY_OP_POSITIVE: return MP_OBJ_NEW_SMALL_INT(value);
-        case RT_UNARY_OP_NEGATIVE: return MP_OBJ_NEW_SMALL_INT(-value);
-        case RT_UNARY_OP_INVERT:
+        case MP_UNARY_OP_BOOL: return o_in;
+        case MP_UNARY_OP_POSITIVE: return MP_OBJ_NEW_SMALL_INT(value);
+        case MP_UNARY_OP_NEGATIVE: return MP_OBJ_NEW_SMALL_INT(-value);
+        case MP_UNARY_OP_INVERT:
         default: // no other cases
             return MP_OBJ_NEW_SMALL_INT(~value);
     }