0.5.0

This is a relatively large release, see #35 for more details.

• Epoch tracking has been removed, along with the Link, AsLink, and Collector::link API surface.
• The Deferred type has also been removed as a consequence of the removal of Link.
• The Clone implementation for Collector has been removed.
• Unprotected guards have been removed.
• Reclaimers now take *mut T, removing the need for type annotations when passing reclaimers to Collector::retire.
• Guard::belongs_to has been removed in favour of a Guard::collector accessor and Collector: PartialEq implementation.
• Reclaimers now receive a reference to the collector as their second argument.
• There are now protected RMW operations on the Guard trait, such as Guard::swap and Guard::compare_exchange.