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_const_eval/src/transform/check_consts/check.rs

@@ -323,7 +323,7 @@ impl<'mir, 'tcx> Checker<'mir, 'tcx> {
         let gate = match op.status_in_item(self.ccx) {
             Status::Allowed => return,
 
-            Status::Unstable(gate) if self.tcx.features().enabled(gate) => {
+            Status::Unstable(gate) if self.tcx.features().active(gate) => {
                 let unstable_in_stable = self.ccx.is_const_stable_const_fn()
                     && !super::rustc_allow_const_fn_unstable(self.tcx, self.def_id(), gate);
                 if unstable_in_stable {