Don't memoize EffectCollection.top_priority_effect()

Closes #131

Also a trivial performance improvement for object equality in Collection and Variant when the objects being compared are the same object.

varcode/collection.py

@@ -89,6 +89,8 @@ def __getitem__(self, idx):
         return self.elements[idx]
 
     def __eq__(self, other):
+        if self is other:
+            return True
         return (
             self.__class__ == other.__class__ and
             len(self) == len(other) and

varcode/effect_collection.py

@@ -178,7 +178,6 @@ def detailed_string(self):
                 lines.append("  Highest Priority Effect: %s" % best)
         return "\n".join(lines)
 
-    @memoize
     def top_priority_effect(self):
         """Highest priority MutationEffect of all genes/transcripts overlapped
         by this variant. If this variant doesn't overlap anything, then this

varcode/variant.py

@@ -219,6 +219,8 @@ def fields(self):
             self.ensembl.release)
 
     def __eq__(self, other):
+        if self is other:
+            return True
         return (
             self.contig == other.contig and
             self.start == other.start and