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/Libraries/LibIPC/Encoder.cpp

@@ -169,7 +169,7 @@ Encoder& Encoder::operator<<(JsonValue const& value)
 
 Encoder& Encoder::operator<<(URL const& value)
 {
-    return *this << value.to_string();
+    return *this << value.to_deprecated_string();
 }
 
 Encoder& Encoder::operator<<(Dictionary const& dictionary)