CFML 2.0 for CakeML

Arthur Charguéraud has developed a new version of CFML based on weakest preconditions.

Here are a few URLs:
- [his course on separation logic ](http://www.chargueraud.org/teach/verif/)
- [his construction of wpgen](http://www.chargueraud.org/teach/verif/slf/LibSepReference.html)
- [the script for a demo](http://www.chargueraud.org/teach/verif/slf/Demo.v)
- [the last 2 pages of this paper explain the new xsimpl tactic](http://www.chargueraud.org/research/2020/seq_seplogic/seq_seplogic.pdf)

This new version seems slicker than the CF definitions and automation used in CakeML's version of CF, which are based on Charguéraud's previous version of CFML.

This issue is about developing a CakeML version of Charguéraud's new CFML.

It would be good if the new CF for CakeML would be compatible with the current CF for CakeML. In particular, it would be good if `app` theorems proved in one are equivalent to `app` theorems proved in the other.

Start by reading the linked course, wpgen construction, Demo.v script, and paper pages describing the xsimpl tactic. Compare these materials with CakeML's current CF definitions and automation. Done means a new CakeML CF is developed and its app theorems are equivalent to those proved with the current CF.

Written by the indexing model from the issue text.