Speed up DeclarativePolicy::Runner#steps_by_score

There were a couple of things here:

1. If the state was already enabled, we don't need to check all the remaining
   steps - only those that can prevent the state. (An enable followed by an
   enable is a no-op.) This logic is in `#run`, but we still did the work of
   scoring and sorting the steps.
2. The sorting is known to be inefficient, but we can make it slightly more
   efficient by stopping once we have a step with zero score, as that means it's
   free.

Neither of these make this _fast_, especially when called lots of times - as we
do when there is lots of activity on an issue - but they do help some.

changelogs/unreleased/declarative-policy-optimisations.yml

+---
+title: Speed up permission checks
+merge_request:
+author:
+type: other

lib/declarative_policy/runner.rb

@@ -107,7 +107,7 @@ module DeclarativePolicy
     end
 
     # This is the core spot where all those `#score` methods matter.
-    # It is critcal for performance to run steps in the correct order,
+    # It is critical for performance to run steps in the correct order,
     # so that we don't compute expensive conditions (potentially n times
     # if we're called on, say, a large list of users).
     #
@@ -139,30 +139,39 @@ module DeclarativePolicy
         return
       end
 
-      steps = Set.new(@steps)
-      remaining_enablers = steps.count { |s| s.enable? }
+      remaining_steps = Set.new(@steps)
+      remaining_enablers, remaining_preventers = remaining_steps.partition(&:enable?).map { |s| Set.new(s) }
 
       loop do
-        return if steps.empty?
-
+        if @state.enabled?
+          # Once we set this, we never need to unset it, because a single
+          # prevent will stop this from being enabled
+          remaining_steps = remaining_preventers
+        else
           # if the permission hasn't yet been enabled and we only have
           # prevent steps left, we short-circuit the state here
-        @state.prevent! if !@state.enabled? && remaining_enablers == 0
+          @state.prevent! if remaining_enablers.empty?
+        end
+
+        return if remaining_steps.empty?
 
         lowest_score = Float::INFINITY
         next_step = nil
 
-        steps.each do |step|
+        remaining_steps.each do |step|
           score = step.score
+
           if score < lowest_score
             next_step = step
             lowest_score = score
           end
-        end
 
-        steps.delete(next_step)
+          break if lowest_score.zero?
+        end
 
-        remaining_enablers -= 1 if next_step.enable?
+        [remaining_steps, remaining_enablers, remaining_preventers].each do |set|
+          set.delete(next_step)
+        end
 
         yield next_step, lowest_score
       end