pull-stream version of fs.createWriteStream
currently really simple and does not yet support all fs.createWriteStream options yet.
var Write =
For large buffers this is currently as fast as node's streams,
but for lots of small buffers it's a little less.
Node uses the fs binding's
writev to pass many buffers to the kernel simutaniously.
TODO: benchmarks with graphs comparing node streams and pull streams.