Edit flatten

[Kodiologist]

• Closes Flattening list of lists of dicts leads to surprising results #3

I've changed flatten a bit but left its behavior largely the same, including returning only the keys of dictionaries. Here's my thinking. The exact way that the user would like to recursively descend into various data structures probably depends on the structure. This function is supposed to be represent a naive approach that just does the simplest useful thing, leaving more specialized notions of flattening to the user. In general, it should be easier to write a new recursive function for specialized cases than to use some customizable interface that we add to flatten.