Everywhere: Rename to_{string => deprecated_string}() where applicable

This will make it easier to support both string types at the same time
while we convert code, and tracking down remaining uses.

One big exception is Value::to_string() in LibJS, where the name is
dictated by the ToString AO.

Userland/Utilities/fgrep.cpp

@@ -25,6 +25,6 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
             TRY(Core::System::write(1, str.bytes()));
         if (feof(stdin))
             return 0;
-        VERIFY(str.to_string().characters());
+        VERIFY(str.to_deprecated_string().characters());
     }
 }