update API using lessons learned from migrating actual hashmap user code
Update 0060-collection-views.md

Make OccupiedEntry.set take &mut self.
Merge pull request #247 from Gankro/entries-update

Merging as per [weekly meeting decision](https://github.com/rust-lang/meeting-minutes/blob/master/weekly-meetings/2014-09-23.md#entry-revisions).