Factor out and more clearly document slicker's model of files

[csilvers]

The parts of slicker that pertain to taking a file (an AST, really), and determining what about it slicker wants to know are currently not really well separated out. These would be nice to separate, and document as such, and perhaps rename some of. Some of the methods could become class methods: _localnames_from_fullnames would become LocalName.from_fullnames or some such. And this would serve as a good place to document overall how it is that slicker thinks about files: in terms of fullnames, localnames, and the like.

[csilvers]

I'd also love a class Import, that holds information about a single import. It could also hold some logic like __str__(), which is a free function right now, or localname(fullname) which says what the localname for a given fullname would be if it used that import.