add some sequence examples to data structures cookbook

[bobisageek]

A small expansion to the data structures cookbook, focused on a few examples that use handful of different sequence functions.

The second example is definitely more contrived, but it's sort of a twist on some exercises that I've seen questions about on the slack (as in, using partition and take-while).

[seancorfield]

Thank you! The site has been regenerated so those changes should show up in a few minutes.