A `Substream` can be read in one of two ways:
* Converting it to an iterator that yields individual chunks of the substream in `Slice` form.
* Converting it to a `SubstreamCursor`, which implements `Read` for bytewise read access.
Chunk-based reading is intended for code that needs to know about chunk boundaries. Self-terminating chunk formats should be read with the cursor type.
This is ultimately intended to replace `Vec<u8>` in both `SwfMovie` and `NetStream`, but for the time being we only apply this to streams. SWF-related types are deeply embedded elsewhere and the changes to slice dereferencing will be quite invasive.
We also remove some unnecessary reader drops in `NetStream.tick` at the same time.
David Wendt