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_passes/src/check_const.rs

@@ -86,7 +86,7 @@ impl<'tcx> CheckConstVisitor<'tcx> {
         let is_feature_allowed = |feature_gate| {
             // All features require that the corresponding gate be enabled,
             // even if the function has `#[rustc_allow_const_fn_unstable(the_gate)]`.
-            if !tcx.features().enabled(feature_gate) {
+            if !tcx.features().active(feature_gate) {
                 return false;
             }
 
@@ -134,7 +134,7 @@ impl<'tcx> CheckConstVisitor<'tcx> {
 
         let required_gates = required_gates.unwrap_or(&[]);
         let missing_gates: Vec<_> =
-            required_gates.iter().copied().filter(|&g| !features.enabled(g)).collect();
+            required_gates.iter().copied().filter(|&g| !features.active(g)).collect();
 
         match missing_gates.as_slice() {
             [] => {