Move collections from libextra into libcollection

Changes:
- Everything labeled under collections in libextra has been moved into a new crate 'libcollection'.
- Crates that depend on the collections have been updated and dependencies sorted.
- I think I changed all the imports in the tests to make sure it works. I'm not entirely sure, as near the end of the tests there was yet another `use` that I forgot to change, and when I went to try again, it started rebuilding everything, which I don't currently have time for. 

There will probably be incompatibility between this and the other pull requests that are splitting up libextra. I'm happy to rebase once those have been merged.

The tests I didn't get to run should pass. But I can redo them another time if they don't.