Add `mkpipe` function

`mkpipe` ("make pipe") is like `compose`, but the functions are applied left-to-right,
like the `pipe` function.

I know this is a trivial addition, logically speaking, but I find it much more natural to think of function composition as applying the functions left-to-right, as a series of operations, rather than right-to-left -- just like Unix pipelines and the toolz `pipe` function. Various libraries and functional languages (e.g., fn.py, F#) make both kinds of function composition available. And for what it's worth, I find myself defining this function in every project I use toolz in. I appreciate that we want to keep the toolz namespace from growing without bound, but this seems to be a natural and useful fit between `compose` and `pipe`.