This also removes the double use of the name 'wp_fork' in both
program_logic/weakestpre and heap_lang/lifting.

Add lock interface

@jung looks good?

See merge request !3

NB: these scopes delimiters were already there before Janno's a0067662.

Following the time anology of later, the step-index 0 corresponds does
not correspond to 'now', but rather to the end of time (i.e. 'last').

Make names more consistent with the rest of the development, make
definitions type classes opaque so that the proofmode does not unfold
then, declare timeless, persistent and proper instances.

unfolded logically atomic triple

The definition is a bit funky now to hack together something fast. See demo section for an example of application.

cc @dreyer @jung @robbertkrebbers @jjourdan 

See merge request !7

This is allowed as long as one of the conjuncts is thrown away (i.e. is a
wildcard _ in the introduction pattern). It corresponds to the principle of
"external choice" in linear logic.

Also, since do_head_step no longer has a purpose, I have removed it
and just use a bunch of eauto hints.

This implements issue #3.

This is more consistent with CAS, which also can be used on any value.
Note that being able to (atomically) test for equality of any value and
being able to CAS on any value is not realistic. See the discussion at
https://gitlab.mpi-sws.org/FP/iris-coq/issues/26, and in particular JH
Jourdan's observation:

  I think indeed for heap_lang this is just too complicated.

  Anyway, the role of heap_lang is not to model any actual
  programming language, but rather to show that we can do proofs
  about certain programs. The fact that you can write unrealistic
  programs is not a problem, IMHO. The only thing which is important
  is that the program that we write are realistic (i.e., faithfully
  represents the algorithm we want to p

This commit is based on a commit by Zhen Zhang who generalized equality
to work on any literal (and not just integers).