About ranges and iterators again: implementing next_permutation

[Max Klyga]

Several weeks ago Andrei mentioned at #d irc channel that he wanted to 
implement next_permutation STL function in Phobos.
I thought that would be a nice excercize to implement it myself.

I ported implementation mentioned in this article: 
http://marknelson.us/2002/03/01/next-permutation/

At first it seemed trivial, but i failed to preserve the same 
constraints that STL implementation requires while preserving same 
efficiency.

A straightforward port using random access ranges and indeces as 
iterators: http://pastie.org/2193659

I tried to implement the same algorithm for bidirectional ranges, but 
it is obviously slower, because i have to shrink range's left side all 
the way, while iterators have a very nice ability: two iterators make a 
range on which algorithms can operate. This is imposible with ranges. 
At least I haven't found a way to do it.

An attempt to implement next_permutation for biderectional ranges: 
http://pastie.org/2193686
This implementation runs about 2-2.5 times slower on my machine.

I think it's a nice challenge for D community to try to adapt this 
algorithm for ranges, allowing it be as fast and as generic as STL 
version.