It's a better name, and lets "active features" refer to the features
that are active in a particular program, due to being declared or
enabled by the edition.
The commit also renames `Features::enabled` as `Features::active` to
match this; I changed my mind and have decided that "active" is a little
better thatn "enabled" for this, particularly because a number of
pre-existing comments use "active" in this way.
Finally, the commit renames `Status::Stable` as `Status::Accepted`, to
match `ACCEPTED_FEATURES`.
