Get rid of the 64 rights limitation

There is currently an hardcoded limitation of 64 rights: we currently never allow to register more rights than that. The rationale behind that limitation is that currently the RightSet implementation uses a 64-bits integer storage for optimization.

A side-effect of that limitation is that in case of registering/unregistering a right, we're currently reusing places in a list of values used internally: we haven't shown it yet, but in theory it's possible that it leads to bugs if for some reason a piece of code tries to reuse the ordinal number of an unregistered right after it has been reassigned to another right.

But besides that side effect, this limitation might become a problem as we use more and more custom rights in extensions.