Clean curried namespace

The dunders from `toolz` and `toolz.curried_exceptions` were being copied to `toolz.curried`.  One of the effects of this is the doctests for `toolz.curried` weren't being run.  If we tried, I'm sure we could find other funcy things that may happen due to copying dunders from one module to another.

So, this PR doesn't copy any dunders to the `toolz.curried` namespace, and it fixes the doctests that previously weren't run.  It also make the variables and functions used to set up `toolz.curried` private by adding the '_' prefix.