Add impls for new std::io::{Read,Write} structs

[fkrull]

I added impls for Debug, PartialEq, Eq, Hash, Clone to the new helper structs.

@sdroege There already are into_input_stream and into_output_stream methods on the structs to turn them back into the gio types. Or did you mean something else?

[sdroege]

@sdroege There already are into_input_stream and into_output_stream methods on the structs to turn them back into the gio types. Or did you mean something else?

@fkrull I was thinking of Deref, or a function to get the stream reference from it (without consuming the adapter). Do you think that would be useful?

Apart from that the PR here looks good. Sorry for not replying earlier!

[fkrull]

I added some methods to get references to the wrapped streams. I decided against using Deref though since it seems semantically incorrect.

[sdroege]

Looks good to me. @GuillaumeGomez ?

[GuillaumeGomez]

Looks good to me as well. Thanks!