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/cmSetPropertyCommand.h

@@ -44,7 +44,7 @@ public:
   cmTypeMacro(cmSetPropertyCommand, cmCommand);
 
 private:
-  std::set<cmStdString> Names;
+  std::set<std::string> Names;
   std::string PropertyName;
   std::string PropertyValue;
   bool Remove;