Improve performance of ReservedSpace

[nolanderc]

Pull Request

Related issue

Addresses (some) concerns raised in a comment: #256 (review).

What does this PR do?

• Marks most methods of ReservedSpace as #[inline] allowing them to be inlined across crate boundaries. Especially for the Write-impl where callers may perform small writes, the overhead of not inlining can be significant.
• See reduced overhead of MDB_RESERVE Kerollmops/heed-perfs-put-reserved#1 for some benchmarks and details.

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?
  • Where can I find these?
• 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.

I looked at the investigation and it seems the fastest implementation is the put-reserved-uninit method. What do you think?

Ho! And BTW, don't worry about the contribution guideline, this template is from the Meilisearch repo 😃

[Kerollmops]

You are really a beast 🐻 That's amazing work!