Serialization

Some small commits to improve function serialization within `toolz`

In particular due to some import issue using `dill` with the curried version of `merge_with` made `dill` want to bring along all of `toolz` in the string representation.  Keeping it in a separate file from `curried.py` fixed the problem.

Compose is now a class, exposing its functions as fields rather than within a function closure.  The API function `compose` still exists.  This is purely an internal change.