More convenience methods on StreamDef

Might as well get some mileage out of Algebird...

And `invert` turns out to be surprisingly useful when you're doing a lot
of merging-and-grouping.
Linear Road example and tests

At this point, this is just a rough sketch... but one has to start
somewhere.
Remove 'windowed' function

I'm not quite ready to approach windowing yet, and there are no use
cases for it in `core`, so dropping it for now.