Roll repr legacy2

1. Do we need compare_is_valid ? It was not in list. If we do not need, please, take a look on proof and question anyway since I have the same question for Level_repr.v I was not able to use Utils.tezos_z_auto to proof compare_is_valid except as instead auto. It is possible I need download example file and examine it more accurate. Please, let me know if I need it for this task.
2. I defined Valid.t just as you can see. I think it should be related with something else, but I'm not sure to what exactly. I tried to see on Level_repr, but didn't get the idea.
3. rpc_arg_valid. @dhilst helped me (thanks!) with

eapply RPC_arg.Valid.like.
apply RPC_arg.Valid.uint31.

but we were not able to move forward. So, I need comments here and with index_path_encoding_is_valid