[Rust] Use concurrent hashmap

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.31%. Comparing base (32f1f77) to head (e125297). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #473 +/- ## ======================================= Coverage 99.31% 99.31% ======================================= Files 128 128 Lines 8369 8369 ======================================= Hits 8312 8312 Misses 57 57 ``` | [Flag](https://app.codecov.io/gh/massquantity/LibRecommender/pull/473/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=massquantity) | Coverage Δ | | |---|---|---| | [CI](https://app.codecov.io/gh/massquantity/LibRecommender/pull/473/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=massquantity) | `99.31% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=massquantity#carryforward-flags-in-the-pull-request-comment) to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.