Close files more quickly, not relying on destructors

[codecov[bot]]

Codecov Report

Patch coverage: 93.33% and project coverage change: -0.03 :warning:

Comparison is base (dedad39) 94.20% compared to head (7ff9d17) 94.17%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #26 +/- ## ========================================== - Coverage 94.20% 94.17% -0.03% ========================================== Files 21 21 Lines 2052 2076 +24 Branches 266 266 ========================================== + Hits 1933 1955 +22 - Misses 119 121 +2 ``` | [Impacted Files](https://app.codecov.io/gh/alugowski/fast_matrix_market/pull/26?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Adam+Lugowski) | Coverage Δ | | |---|---|---| | [include/fast\_matrix\_market/fast\_matrix\_market.hpp](https://app.codecov.io/gh/alugowski/fast_matrix_market/pull/26?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Adam+Lugowski#diff-aW5jbHVkZS9mYXN0X21hdHJpeF9tYXJrZXQvZmFzdF9tYXRyaXhfbWFya2V0LmhwcA==) | `95.74% <ø> (ø)` | | | [python/src/fast\_matrix\_market/\_\_init\_\_.py](https://app.codecov.io/gh/alugowski/fast_matrix_market/pull/26?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Adam+Lugowski#diff-cHl0aG9uL3NyYy9mYXN0X21hdHJpeF9tYXJrZXQvX19pbml0X18ucHk=) | `93.50% <93.33%> (-0.22%)` | :arrow_down: |

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