Enhance performance

Instead of sorting the whole array of elements in each iteration of the
huffman tree construction, we now search backwards for the first index
where the sum to insert is equal to or greater than the weight at that
index. Directly after that index the sum is inserted. According to some
tests I made, this is about 3 times faster than before.
Merge branch 'huffman' of git://github.com/lydell/VimFx into lydell-huffman