Merge: No intrude rope stream

Two things happening here, in two commits:

* the first commit moves the definition of `decoder` into Stream, and renames it to `codec` since both coding/decoding will be used by streams when dealing with String semantics.
* the second commit removes the `intrude inport ropes` from stream as this was not necessary, and performance-wise did not matter sufficiently to justify this

Pull-Request: #2632
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>