Don't implement event::Source for all type DerefMut<Target = event::Source>
Don't implement event::Source for all type DerefMut<Target = event::Source>

This prevents wrapper types to implement DerefMut and event::Source,
while the inner type also implement event::Source as that will result in
conflicting trait implementations. Perhaps when specialisation is stable
this can be reverted.

This is required to implement DerefMut for IoSource.

Effectively reverts commit 8397102dc08711a733c62d55ba04cb7d99215c41.

carllerche/mio,carllerche/mio