Rewrite `Lib_usage`

[Halbaroth]

This issue is a remainder to rewrite the example Lib_usage after merging #1251. This module documented how to use the Alt-Ergo library with the legacy frontend. We cannot rewrite it now with Dolmen because a large part of the library logic with the Dolmen frontend is located in the Alt-Ergo binary and should be moved in the library. Writing a proper API for the Alt-Ergo library with the Dolmen AST as input has to be completed before explaining how to use it!