There was not really a need for the lattice type classes, so I removed
these.

These just make things more complicated, it would be more useful to
declare (efficient) decision procedures for each instance, so that
we can properly predict which instance we will get.

This class whose name is horrible and purpose is arbitrary seems to be a
leftover of some experiment with ch2o, a long time a ago.

Thanks to Amin Timany for the suggestion.

It now traverses terms at most once, whereas the setoid_rewrite
approach was travering terms many times. Also, the tactic can now
be extended by defining type class instances.

Also, use "set_solver by tac" to specify a tactic.

In most cases there is a lot of duplicate proof search performed by
both naive_solver and eauto. Especially since naive_solver calls its
tactic (in the case of set_solver this used to be eauto) quite eagerly
this made it very slow.

Note that set_solver is this too slow and should be improved.

In most cases there is a lot of duplicate proof search performed by
both naive_solver and eauto. Especially since naive_solver calls its
tactic (in the case of set_solver this used to be eauto) quite eagerly
this made it very slow.

Note that set_solver is this too slow and should be improved.

It is doing much more than just dealing with ∈, it solves all kinds
of goals involving set operations (including ≡ and ⊆).

simplify_equality        => simplify_eq
simplify_equality'       => simplify_eq/=
simplify_map_equality    => simplify_map_eq
simplify_map_equality'   => simplify_map_eq/=
simplify_option_equality => simplify_option_eq
simplify_list_equality   => simplify_list_eq
f_equal'                 => f_equal/=

The /= suffixes (meaning: do simpl) are inspired by ssreflect.

Also, make our redefinition of done more robust under different
orders of Importing modules.

This one (previously solve_elem_of) was hardly used. The tactic that uses
naive_solver (previously esolve_elem_of, now solve_elem_of) has been
extended with flags to say which hypotheses should be cleared/kept.

* Remove the order from RAs, it is now defined in terms of the ⋅ operation.
* Define ownership using the step-indexed order.
* Remove the order also from DRAs and change STS accordingly. While doing
  that, I changed STS to no longer use decidable token sets, which removes the
  requirement of decidable equality on tokens.