Reduce number of temporary std::strings

Reduces fannkuch.py runtime on my machine from 25.5 secs to 16.5 secs.

src/runtime/objmodel.cpp

@@ -372,10 +372,13 @@ void HCBox::giveAttr(const std::string& attr, Box* val) {
 }
 
 void HCBox::setattr(const std::string& attr, Box* val, SetattrRewriteArgs *rewrite_args, SetattrRewriteArgs2 *rewrite_args2) {
-    RELEASE_ASSERT(attr != "None" || this == builtins_module, "can't assign to None");
+    static const std::string none_str("None");
+    static const std::string getattr_str("__getattr__");
+    static const std::string getattribute_str("__getattribute__");
 
-    bool isgetattr = (attr == "__getattr__" || attr == "__getattribute__");
-    if (isgetattr && this->cls == type_cls) {
+    RELEASE_ASSERT(attr != none_str || this == builtins_module, "can't assign to None");
+
+    if ((this->cls == type_cls) && (attr == getattr_str || attr == getattribute_str)) {
         // Will have to embed the clear in the IC, so just disable the patching for now:
         rewrite_args = NULL;
         rewrite_args2 = NULL;

src/runtime/types.cpp

@@ -246,10 +246,14 @@ extern "C" {
 }
 
 extern "C" Box* createSlice(Box* start, Box* stop, Box* step) {
+    static const std::string start_str("start");
+    static const std::string stop_str("stop");
+    static const std::string step_str("step");
+
     BoxedSlice *rtn = new BoxedSlice(start, stop, step);
-    rtn->setattr("start", start, NULL, NULL);
-    rtn->setattr("stop", stop, NULL, NULL);
-    rtn->setattr("step", step, NULL, NULL);
+    rtn->setattr(start_str, start, NULL, NULL);
+    rtn->setattr(stop_str, stop, NULL, NULL);
+    rtn->setattr(step_str, step, NULL, NULL);
     return rtn;
 }