public String lookup(String str){
        State s = getState();
        s.lookup(str);
        return s.dataString();
    }
Convenience method that returns the metadata string in the fsa
for the input lookup String, or null if the input string does
not exist in the fsa.
@param str The string to look up.
@return Metadata string from the fsa.
fsa/src/main/java/com/yahoo/fsa/FSA.java