ListT proposal for `transformers`

[Gabriel Gonzalez]

I've been working on a "ListT done right" for submission into 
`transformers`.  I could split it off into its own package, but I would 
like to first run it by you all, particularly Ross, to see if this can 
be included this in `transformers` because I feel that `transformers` is 
where this belongs.

I've lpasted the implementation here:

http://lpaste.net/90890

I've written it to follow the spirit of the `transformers` library as 
closely as possible, both in API, documentation, and source code style.

I'm fine with either replacing the old `ListT` implementation or 
providing both.  I'll leave it to you guys to make the call on that.  As 
long as the correct version is in `transformers` somewhere then I'm 
happy.  If I *had* to pick a side, I suppose I would favor replacing the 
old one.

Also, any suggestions for changes are welcome, particularly if they 
improve the likelihood of this getting into `transformers`.