Add str + unicode concat support

src/runtime/str.cpp

@@ -290,9 +290,15 @@ extern "C" PyObject* PyString_FromFormat(const char* format, ...) noexcept {
     return ret;
 }
 
-extern "C" BoxedString* strAdd(BoxedString* lhs, Box* _rhs) {
+extern "C" Box* strAdd(BoxedString* lhs, Box* _rhs) {
     assert(lhs->cls == str_cls);
 
+    if (_rhs->cls == unicode_cls) {
+        Box* rtn = PyUnicode_Concat(lhs, _rhs);
+        checkAndThrowCAPIException();
+        return rtn;
+    }
+
     if (_rhs->cls != str_cls) {
         raiseExcHelper(TypeError, "cannot concatenate 'str' and '%s' objects", getTypeName(_rhs));
     }

test/tests/unicode_test.py

@@ -27,6 +27,8 @@ print u'a' in c.__dict__
 print u'' == ''
 print '' == u''
 print hash(u'') == hash('')
+print "Hello " + u" World"
+print u"Hello " + " World"
 
 try:
     hasattr(object(), u"\u0180")