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/cpp-preprocessor.cpp

@@ -37,7 +37,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
     }
 
     for (auto& token : tokens) {
-        outln("{}", token.to_string());
+        outln("{}", token.to_deprecated_string());
     }
 
     return 0;