Store contraints in a hash structure instead of a map

Our key type here is a vector of fields and vectors aren't less than
comparable by default (and I have no idea how to implement a sane
operator<() for them). But because maps require less than comparability,
change the map to a hash which only requires an operator==() which a
vector should have by default.

No idea how this ever worked with a map?!

See issue #854.

src/sqlitetypes.h

@@ -9,7 +9,7 @@
 #include <QVector>
 #include <QStringList>
 #include <QPair>
-#include <QMultiMap>
+#include <QMultiHash>
 
 namespace sqlb {
 
@@ -155,7 +155,7 @@ private:
     bool m_unique;
 };
 
-typedef QMultiMap<FieldVector, ConstraintPtr> ConstraintMap;
+typedef QMultiHash<FieldVector, ConstraintPtr> ConstraintMap;
 
 class Table
 {