Add proximity calculation for the same word

[msvaljek]

Pull Request

Related issue

Fixes #647

What does this PR do?

• During the increase of the current word position we extract the proximity between the current position and the next one.

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)?
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Thank you so much for contributing to Meilisearch!

[meili-bot]

This message is sent automatically

Hello @msvaljek,
Thank you very much for contributing to Meilisearch ❤️.
However, the team is not available on the weekend, but they will be back on Monday 😊

[ManyTheFish]

Hello @msvaljek, thank you for your contribution!
I let bors check your PR, and if everything goes well it will be automatically merged.

bors merge

[bors]

Build succeeded:

• Run Rustfmt
• Tests on macos-latest
• Tests on ubuntu-20.04
• Tests on windows-latest

[meili-bot]

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