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/dd.cpp

@@ -54,7 +54,7 @@ static int handle_io_file_arguments(int& fd, int flags, StringView argument)
         return -1;
     }
 
-    fd = open(value.to_string().characters(), flags, 0666);
+    fd = open(value.to_deprecated_string().characters(), flags, 0666);
     if (fd == -1) {
         warnln("Unable to open: {}", value);
         return -1;