Feature request: open multiple streams at once

[surban]

Currently it is not possible to have multiple streams open at once, since Stream owns the mutable reference to CompoundFile. This is a bit sad, since it limits the ability of rust-cfb to be used as a virtual filesystem with multiple open files. I understand that implementing this might be a significant challenge, but maybe it would be possible to have multiple read-only streams open at once as a first step?