Using traits ans structures from the standard library

[ghost]

capnproto-rust redefines some traits / structures that seem to exist in the standard library, such as BufferedReader and BufferedWriter and the corresponding wrappers, but with a few changes.

Is there a particular reason to redefine those traits and structures, such as an insufficent API or too much API instability? Or is it there more for legacy reasons?

[dwrensha]

The standard library's API is insufficient. The packing algorithm in serialize_packed makes heavy use of the BufferedInputStream::get_read_buffer() and BufferedOutputStream::get_write_buffer() methods to directly access the buffered bytes.

I'd like to use a safer and more idiomatic API here, but it's unclear whether that's possible without sacrificing performance.

[dwrensha]

Thanks to #41, we now do use the standard traits.