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_target/src/spec/abi.rs

@@ -182,7 +182,7 @@ pub fn is_enabled(
 ) -> Result<(), AbiDisabled> {
     let s = is_stable(name);
     if let Err(AbiDisabled::Unstable { feature, .. }) = s {
-        if features.enabled(feature) || span.allows_unstable(feature) {
+        if features.active(feature) || span.allows_unstable(feature) {
             return Ok(());
         }
     }