Expose bump allocator in Sender

[rj00a]

This PR removes the 'static requirement from Sender::send and Sender::send_to and exposes the bump allocator by providing allocation methods on Sender. Sender is now entirely internally mutable (takes &self on all the methods).

This partially addresses #44 but is missing a few things.

• Doesn't allow received events with borrowed data to pass though Sender::send without a clone because Receiver and Sender have different lifetimes. Might still need a combined Sender and Receiver type.
• Can't use the bump allocator from a &mut World.
• No optional collections module. I think it would be easier to just add an unstable feature for the unstable allocator api for those who need it.