core/stream: change read_byte return type to Int

core/stream: change read_byte return type to Int

A read_byte used to return a nullable Byte, which could cause
unnecessary boxings, and thus could harm performance.

This commit changes its return to Int, and adopts a closer-to-C api,
returning a negative value when an error occurs.

Signed-off-by: Lucas Bajolet <lucas.bajolet@gmail.com>