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.

It now first turns hypotheses `X ∪ Y ⊆ Z` into `X ⊆ Z` and `Y ⊆ Z`.

Add IntoPure instances for ∗, ∧, and ∨.

See merge request !49

Add a contributor's guide

See merge request !47