* 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.

The port makes the following notable changes:

* The carrier types of separation algebras and integer environments are no
  longer in Set. Now they have a type at a fixed type level above Set. This
  both works better in 8.5 and makes the formalization more general.
  I have tried putting them at polymorphic type levels, but that increased the
  compilation time by an order of magnitude.
* I am using a custom f_equal tactic written in Ltac to circumvent bug #4069.
  That bug has been fixed, so this custom tactic can be removed when the next
  beta of 8.5 is out.

It would still be far more efficient to have a counter for the next memory
index in the executable semantics/frontend.

Ported from popl2014 branch.

First it would destruct on the decider, which sometimes would result
in unfolded hypotheses.

The tactic "injection' H" now uses the name "H" for the first hypothesis it
generates. Fresh names will still be used for the remaining hypotheses.

Important changes in the core semantics:
* Types extended with function types. Since function types are a special kind
  of pointer types, types now have an additional mutual part called "ptr_type".
* Pointers extended with function pointers. Theses are just names that refer
  to an actual function in the function environment.
* Typing environments extended to assign argument and return types to function
  names. Before we used a separate environment for these, but since the
  argument and return types are already needed to type function pointers, this
  environment would appear in pretty much every typing judgment.

As a side-effect, the frontend has been rewritten entirely. The important
changes are:

* Type checking of expressions is more involved: there is a special kind of
  expression type corresponding to a function designator.
* To handle things like block scoped extern function, more state-fullness was
  needed. To prepare for future extensions, the entire frontend now uses a
  state monad.