Remove the generic types of the RoTxn and RwTxn

[Kerollmops]

We introduce a generic type to the RoTxn and RwTxn structs to make sure that we don't use a transaction created on one environment with another environment, this introduced more methods on the Env struct to create typed transactions.

I am not quite sure it makes sense now that we added a runtime check to make sure the transactions are valid and match the same environment.