* New feature <relevant> which is automatically deduced in most cases.
* Features which are not relevant do not affect target paths and
do not prevent merging of virtual targets.
* generators.jam: generator.run always returns usage-requirements
to allow usage-requirements in non-top-level generators. This
is necessary because we're using usage-requirements to track
relevance from flags.
* New rule toolset.uses-features to specify features that the rule
checks manually, instead of using toolset.flags. In the future,
we should consider restricting the properties actually passed,
to detect errors.
* Adjust tests to handle the new paths (verified by inspection).
* Add temporary option --ignore-relevance to consider all features relevant
to aid migration.
* New rule property.evaluate-conditional-relevance which helps tracking
relevance in <conditional>.
* Widely scattered changes to use the new interfaces.
