Add ReadWrapper and WriteWrapper traits

[bozaro]

I wrote simple binding for lz4 compression (https://github.com/bozaro/lz4-rs) and got issue bozaro/lz4-rs#9.

The main problem is: there are not best practices for writing compression (encryption and etc) libraries.
Compression libraries need some method for "finish" work: write end stream mark and flush data.
This work should not run in drop() method: I prefer unexpected end of stream instead of success on reading incomplete stream.

@kali have had a look at other implementations (std::io::BufWriter, flate2, snappy_framed, which are the ones I need to switch). The four of them are handling thing differently:

• snappy http://docs.randomhacks.net/snappy_framed-rs/snappy_framed/write/struct.SnappyFramedEncoder.html expects the user to call flush(), and do not allow inner writer retrieval
• flate2 offers finish(), but manage the inner ownership internally with an option
• stdlib BufWriter goes even a bit further by handling a possible panic during the finish()

So, I think, in this case good idea to add ReadWrapper and WriteWrapper traits to rust stdlib.

[rust-highfive]

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[durka]

Seems like this should start as an RFC.

[alexcrichton]

Yes @durka is correct here, for addition to the standard library new traits like this require an RFC. I'm gonna close this for now, but let me know if you need any help writing an RFC!

[bozaro]

I try with RFC: rust-lang/rfcs#1568