Added two proper instances with permutations

See merge request !76

This closes issue #36.

show a Proper instance for dom

See merge request !74

make solve_ndisj more powerful

Closes #34

See merge request !75

add unfolding lemma for bool_decide: bool_decide_decide

See merge request !73

Some missing results about vectors.

See merge request !71

Strings are inhabited

See merge request !70

Fix typo in instance name

See merge request !69

Now we follow Coq's stdlib and declare this instance using a `Hint Extern`;
this avoids making `flip` type class opaque.

Revert "`RelDecision` instance for `flip`, and make `flip` tc opaque to avoid loops due to eager unification."

This reverts commit b81aa3aa.

`RelDecision` instance for `flip`, and make `flip` tc opaque to avoid loops due to eager unification.

Otherwise type class search ocasionally unfolds them and finds
wrong instances. Based on an issue reported by @jihgfee.