Place collections into the root namespace of collection crate

[imv]

Currently fully-qualified names of collections contain the name of the collection twice, e.g. collections::btree::BTree. This extra btree does not appear to serve any purpose. There would be no name collisions if all collections are placed directly into the root namespace, as all functionality is implemented as methods. The sub-modules could be left for things like collections::dlist::Items, whereas collections::dlist::DList should become just collections::DList.

[bluss]

This is already the case. (By the way collections as a crate is a subpart of libstd, and thus you should use it through std::collections if you can.)

Doc links:

• http://doc.rust-lang.org/std/collections/
• http://doc.rust-lang.org/collections/index.html