refactor(io): separate types for read and write streams

This is a structural refactor with no logical changes, yet. Done in
preparation for simplifying rstream/rbuffer which will require more
state inline in RStream.

The initial idea was to have RStream and WStream as sub-types
symetrically but that doesn't work, as sockets are both reading and
writing. Also there is very little write-specific state to start with,
so the benefit of a separate WStream struct is a lot smaller. Just
document what fields in `Stream` are write specific.

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