This commit removes the internal hash map from Windows' Selector implementation
keeping track of what to do after any I/O completes. The "completion callback"
is now stored right after the OVERLAPPED structure in memory, and unsafe casting
is done to get back to the original `Arc<T>` and process the I/O event.

Currently this involves vendoring a small copy of `Arc<T>` to ensure that the
memory layout is guaranteed, but this guarantee could in theory be added to the
standard library one day.

Closes #243