Rename ACTIVE_FEATURES as UNSTABLE_FEATURES.

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`.

compiler/rustc_feature/src/accepted.rs

@@ -7,7 +7,7 @@ macro_rules! declare_features {
     ($(
         $(#[doc = $doc:tt])* (accepted, $feature:ident, $ver:expr, $issue:expr, None),
     )+) => {
-        /// Those language feature has since been Accepted (it was once Active)
+        /// Formerly unstable features that have now been accepted (stabilized).
         pub const ACCEPTED_FEATURES: &[Feature] = &[
             $(Feature {
                 name: sym::$feature,