Implement getBytes()

include/VersionedMap.h

@@ -170,7 +170,8 @@ struct VersionedMap {
   /** The "past-the-end" iterator. */
   Iterator end(int64_t version) const;
 
-  /** Returns the memory usage in bytes. */
+  /** Returns the memory usage in bytes. Does not include memory used by
+   * iterators. */
   int64_t getBytes() const;
 
   /** Map starts with no mutations, with `getOldestVersion()` == `getVersion()`