make ReservedSpace more flexible

[nolanderc]

Pull Request

Related issue

Fixes #254

What does this PR do?

• Adds a few methods to ReservedSpace which grant access to the underlying bytes without having to go through the Write impl.
• Adds impl std::io::Seek for ReservedSpace which allows seeking within the reserved space and thus re-write parts of the space already written.

PR checklist

Please check if your PR fulfills the following requirements:

• Does this PR fix an existing issue, or have you listed the changes applied in the PR description (and why they are needed)?
• Have you read the contributing guidelines?
• Have you made sure that the title is accurate and descriptive of the changes?

Thank you so much for contributing to Meilisearch!

[Kerollmops]

Thank you very much for your work!

However, this ReservedSpace struct and API do not seem as optimized and performance optimal as I wanted. I created a small reproduction of the issue and an example of usage at Meilisearch. Would you mind taking some time to see if you can find the design issue in this API?

[Kerollmops]

LGTM 👏 Merging this one for v0.20.1. Thank you again!