* 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.
Various restructure of test framework handling of paths to avoid duplicate path editing which caused failed tests. Adjust many tests to deal with added subdirectories in build outputs now that toolset requirements are active.
