io: stream-contents, -seekable?, -length generics

io: stream-contents, -seekable?, -length generics
stream-contents is extremely slow on decoder streams when implemented with each-block, and it can be implemented very quickly for streams of known length using stream-read-unsafe. Make it generic and provide some off-the-shelf implementations using each-block, a read1 loop, or length + read-unsafe. Provide new stream-seekable? and stream-length generics that can be implemented by stream implementations that know their size.