[varLib.featureVars] Optimize popCount()

While might look like unjustified micro-optimizing, for featureVars_test.py with
n=100, speeds up 10%.  From 2.5s to 2.2s.

Lib/fontTools/varLib/featureVars.py

@@ -73,13 +73,13 @@ class hashdict(dict):
         return result
 
 def popCount(v):
-    # TODO Speed up
-    i = 0
-    while v:
-        if v & 1:
-            i += 1
-        v = v >> 1
-    return i
+    if v > 0xFFFFFFFF:
+        return popCount(v >> 32) + popCount(v & 0xFFFFFFFF)
+
+    # HACKMEM 169
+    y = (v >> 1) & 033333333333
+    y = v - y - ((y >> 1) & 033333333333)
+    return (((y + (y >> 3)) & 030707070707) % 077)
 
 
 def addFeatureVariations(font, conditionalSubstitutions):