strings: Remove cmStdString references

Casts from std::string -> cmStdString were high on the list of things
taking up time. Avoid such implicit casts across function calls by just
using std::string everywhere.

The comment that the symbol name is too long is no longer relevant since
modern debuggers alias the templates anyways and the size is a
non-issue since the underlying methods are generated since it's
inherited.

Source/cmVariableWatch.h

@@ -81,7 +81,7 @@ protected:
   };
 
   typedef std::vector< Pair* > VectorOfPairs;
-  typedef std::map<cmStdString, VectorOfPairs > StringToVectorOfPairs;
+  typedef std::map<std::string, VectorOfPairs > StringToVectorOfPairs;
 
   StringToVectorOfPairs WatchMap;
 };