Change AuReader to read from Read without Seek

[karip]

Currently, creating AuReader with AuReader::new() requires the inner reader to implement std::io::Read and std::io::Seek.AuReader needs to be changed to support inner readers implementing std::io::Read only. However, it should also keep the current functionality for inner readers implementing std::io::Seek.

Therefore, this change seems to require some kind of specialization. The AuReader::new() function should behave slightly differently depending on the inner reader:

• if the inner reader implements std::io::Read only, then initial_stream_len should be set to None.
• if the inner reader implements std::io::Read and std::io::Seek, then initial_stream_len should be set to Some(stream_length).

Rust doesn't seem to support specialization at the moment, so it isn't possible to have two different behaviors for new().

There's private AuReader::new_read() as an example how the new() function would look like for inner readers implementing only std::io::Read, but it can't be used, because calling it for inner readers implementing std::io::Seek would give wrong results (initial_stream_len would be set to None instead of Some(..)).