Clean up transitionary glue in task/spawn.rs. Don't hold kill-little-…

…lock for O(n) time, cf rust-lang#3100, and optimize out several unneeded clone()s.

src/libstd/rt/kill.rs

@@ -488,8 +488,8 @@ impl Death {
         rtassert!(self.unkillable == 0);
         self.unkillable = 1;
 
-        // FIXME(#7544): See corresponding fixme at the callsite in task.rs.
-        // NB(#8192): Doesn't work with "let _ = ..."
+        // NB. See corresponding comment at the callsite in task.rs.
+        // FIXME(#8192): Doesn't work with "let _ = ..."
         { use util; util::ignore(group); }
 
         // Step 1. Decide if we need to collect child failures synchronously.

src/libstd/rt/task.rs

@@ -253,12 +253,10 @@ impl Task {
             }
         }
 
-        // FIXME(#7544): We pass the taskgroup into death so that it can be
-        // dropped while the unkillable counter is set. This should not be
-        // necessary except for an extraneous clone() in task/spawn.rs that
-        // causes a killhandle to get dropped, which mustn't receive a kill
-        // signal since we're outside of the unwinder's try() scope.
-        // { let _ = self.taskgroup.take(); }
+        // NB. We pass the taskgroup into death so that it can be dropped while
+        // the unkillable counter is set. This is necessary for when the
+        // taskgroup destruction code drops references on KillHandles, which
+        // might require using unkillable (to synchronize with an unwrapper).
         self.death.collect_failure(!self.unwinder.unwinding, self.taskgroup.take());
         self.destroyed = true;
     }

src/libstd/task/mod.rs

@@ -38,7 +38,6 @@
 use prelude::*;
 
 use cell::Cell;
-use cmp::Eq;
 use comm::{stream, Chan, GenericChan, GenericPort, Port};
 use result::Result;
 use result;