add row filter for string ==

This gets the census `== 'Dawson Creek'` query down to ~410ms from
~650ms.

That still seems much slower than it should be. Am I accidentally
doing a copy? Now to go learn how to profile C++ code...

parquet/parquet_filter.cc

@@ -39,10 +39,10 @@ double Constraint::getDouble() {
   return doubleValue;
 }
 
-const std::vector<unsigned char> Constraint::getBytes() {
+const std::vector<unsigned char>& Constraint::getBytes() {
   return blobValue;
 }
 
-std::string Constraint::getString() {
+const std::string& Constraint::getString() {
   return stringValue;
 }