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_lint/src/expect.rs

@@ -11,7 +11,7 @@ pub(crate) fn provide(providers: &mut Providers) {
 }
 
 fn check_expectations(tcx: TyCtxt<'_>, tool_filter: Option<Symbol>) {
-    if !tcx.features().enabled(sym::lint_reasons) {
+    if !tcx.features().active(sym::lint_reasons) {
         return;
     }