fs: make `SyncWriteStream` inherit from `Writable`

Make the internal `SyncWriteStream` a proper `stream.Writable`
subclass. This allows for quite a bit of simplification, since
`SyncWriteStream` predates the streams2/streams3 implementations.

Fixes: https://github.com/nodejs/node/issues/8828
PR-URL: https://github.com/nodejs/node/pull/8830


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>