Port to Coq 8.5 beta 2.

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.