Introduce a new `stream` module to `radicle::cob`. The purpose of this module is
to provide an API for iterating over a COB's operations, given a range of
commits.
The `CobStream` trait provides the generic API for implementing and using, while
`Stream` provides a concrete implementation using the `git2` crate.
The approach is to use a `git2::Revwalk` for walking over each commit in the
range, then loading the `Op` for that commit – skipping any commits that are
unrelated to that COB type, in the case of associated COBs.
The `Stream` is tested using some properties of the API.
The next steps are to introduce specialised types for `Patch` and `Issue` to
ensure that it works for those.
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