Also, make the setup of IntoLaterN more consistent with that of WandWeaken.
Maybe, we should do something similar for other proof mode classes too.

Now, we never need to unfold LimitPreserving in LambdaRust, and hence
the entails_lim tactic is no longer needed.

There is no need to restrict the type class using Hint Mode, we have
a default instance that will always be used first. In case of evars,
the default instance should apply.

The reason for this change is that `iAssumption` should be able to
prove `H : ?e |- P` and `H : P |- ?e`. The former Hint Mode prevented
it from doing that.

I probably tested against a wrong version.

This reverts commit 02bc52b4.

This makes lambdarust 1 min (=7%) faster.

This fixes issue #72.