Clean up heap_lang and remove some FIXMEs.

Notable changes:
* I am now using the same names for the fields of the language record and the
  instances in heap_lang. In order to deal with shadowing, I have put all
  definitions in heap_lang.v in a module.
* Instead of defining evaluation contexts recursively, these are now defined
  using lists. This way we can easily reuse operations on lists. For example,
  composition of evaluation contexts is just appending lists. Also, it allowed
  me to simplify the rather complicated proof of step_by_val as induction on
  the shape of contexts no longer results in a blow-up of the number of cases.
* Use better automation to prove all lemmas of heap_lang.
* I have introduced tactics to invert steps and to do steps. These tactics
  greatly helped simplifying boring parts of lifting lemmas.