Add Extract trait

[Thomasdezeeuw]

Because of the way that iouring works the kernel needs mutable access to the
input to a systemcall for entire duration the operation is in progress. For
example when reading into a buffer the buffer needs to stay alive until the
kernel has written into it, or until the kernel knows the operation is
canceled and won't write into the buffer anymore. If we can't ensure this
the kernel might write into memory we don't own causing write-after-free
bugs.

To ensure the input stays alive A10 needs ownership of the input arguments
and leaks the inputs when a Future operation is dropped before
completion. However to give the Futures a nice API we don't return the
input arguments and try to match the API that don't take ownership of
argument, e.g fs::File::open just returns a File not the path
argument.

In some cases though we would like to get back the input arguments from the
operation, e.g. for performance reasons. The Extract trait allow you to do just
that: extract the input arguments from Future operations.