Prepare 0.4.0 release

Signed-off-by: Patrick Roy <roypat@amazon.co.uk>

Changed

- Updated to Rust version 2021.
- Dependencies are now specified by caret, so that they're no longer
  automatically updated when new ones are published.
- The `Display` implementations for the `EventFd` and `Epoll` variants of the
  `Error` type now contain the inner error message as well.

Added

- The `Error` type now implements `Eq`.

v0.2.1

Changed
- Updated the vmm-sys-util dependency to v0.8.0.

Fixed
- Fixed `RemoteEndpoint` `Clone` implementation.
- Check the maximum capacity when calling `EventManager::new`.

Fixed

- Fixed a race condition that might lead to wrongfully call the dispatch
  function for an inactive event
  ([[#41]](#41)).

Added

- By default, the event manager can dispatch 256 events at one time. This limit
  can now be increased by using the `new_with_capacity` constructor
  ([[#37]](#37)).

v0.1.0

This is the first event-manager release.

Read more about the features included in this crate in the
[design document](https://github.com/rust-vmm/event-manager)