* The base registry implementation has been moved to a new

  `logilab.common.registry` module (see #1916014). This includes code from :

  * `cubicweb.vreg` (the whole things that was in there)

  * `cw.appobject` (base selectors and all).

  In the process, some renaming was done:

  * the top level registry is now `RegistryStore` (was `VRegistry`), but that

    should not impact cubicweb client code ;

  * former selectors functions are now known as "predicate", though you still use

    predicates to build an object'selector ;

  * for consistency, the `objectify_selector` decoraror has hence be renamed to

    `objectify_predicate` ;

  * on the CubicWeb side, the `selectors` module has been renamed to

    `predicates`.

  Debugging refactoring dropped the more need for the `lltrace` decorator.

  There should be full backward compat with proper deprecation warnings.

  Notice the `yes` predicate and `objectify_predicate` decorator, as well as the

  `traced_selection` function should now be imported from the

  `logilab.common.registry` module.