io_splice 1.0.0

io_splice is now easier to use, as it can take IO/File objects in
addition to raw file descriptors as arguments.

IO.vmsplice now always splices buffers in full, since it's too difficult
to use otherwise.  Keep in mind that the usefulness of vmsplice is still
questionable from Ruby and IO.writev (from the "io-extra" gem) is more
flexible as it allows writing to non-pipe descriptors.

There are also some code cleanups and improved documentation.