The management of the join contexts now relies on the `Query` object,
that has all the necessary logic to do so. The refactoring also removes
the class `ExtendedLeaf`, which is no longer necessary: the join
contexts are all managed by a single `Query` object, and the parsing
stack now only contains triples like (leaf, model, table_alias).
Also inline directly the SQL translation made in `_to_sql`, which
overall simplifies the management of SQL parameters in the domain
compilation. The algorithm is illustrated in the code itself.
Raphael Collet
84a595301e