Pass input string by const reference

We're not mutating the string, so it can be const. And we can pass
by reference instead of by value to avoid the copy.

zuul-preview/main.cc

@@ -23,7 +23,7 @@
 
 using namespace std;
 
-vector<string> split(string in)
+vector<string> split(const string &in)
 {
   istringstream stream(in);
   vector<string> parts;