Replace NSMutableSet with NSHashTable when Appropriate #trivial (#321)

* Use NSHashTable to avoid needless -hash and -isEqual: calls

* Mark debug-only methods as such for clarity

* Address feedback

Source/Base/ASBaseDefines.h

@@ -235,6 +235,20 @@
   s; \
 })
 
+/**
+ * Create a new ObjectPointerPersonality NSHashTable by mapping `collection` over `work`, ignoring nil.
+ */
+#define ASPointerTableByFlatMapping(collection, decl, work) ({ \
+  NSHashTable *t = [NSHashTable hashTableWithOptions:NSHashTableObjectPointerPersonality]; \
+  for (decl in collection) {\
+    id result = work; \
+    if (result != nil) { \
+      [t addObject:result]; \
+    } \
+  } \
+  t; \
+})
+
 /**
  * Create a new array by mapping `collection` over `work`, ignoring nil.
  */