Bitmap: Add Bitmap::find_next_range_of_unset_bits()

AK::Bitmap is extended with find_next_range_of_unset_bits().
The function is implemented using count_trailing_zeroes_32(), which is
optimized on many platforms, that gives a huge performance boost.
Functions find_longest_range_of_unset_bits() and find_first_fit() are
implemented with find_next_range_of_unset_bits(). According to
benchmarks, they are 60-100% faster.

AK/Platform.h

@@ -97,4 +97,4 @@ template<typename T>
         }
         return 0;
 #endif
-}
+}