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/syscall.cpp

@@ -128,7 +128,7 @@ static FlatPtr as_buf(Vector<FlatPtr> params_vec)
             builder.appendff(" {:p}", params_vec[i]);
         }
         builder.appendff(" ] at {:p}", (FlatPtr)buf);
-        dbgln("{}", builder.to_string());
+        dbgln("{}", builder.to_deprecated_string());
     }
 
     // Leak the buffer here. We need to keep it until the special syscall happens,