Fix std::slice::from_raw_parts usage from Nightly

[PierreZ]

Fix #121

[coveralls]

Pull Request Test Coverage Report for Build 7892530961

Details

• -7 of 23 (69.57%) changed or added relevant lines in 4 files are covered.
• 27 unchanged lines in 5 files lost coverage.
• Overall coverage decreased (-0.4%) to 74.017%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
foundationdb/src/mapped_key_values.rs	5	12	41.67%

Files with Coverage Reduction	New Missed Lines	%
foundationdb/src/directory/directory_layer.rs	2	94.68%
foundationdb/src/mapped_key_values.rs	2	45.87%
foundationdb/src/directory/mod.rs	3	92.56%
foundationdb-bindingtester/src/main.rs	5	87.9%
foundationdb/src/directory/directory_partition.rs	15	77.42%

Totals
Change from base Build 7633321970:	-0.4%
Covered Lines:	4199
Relevant Lines:	5673

---

💛 - Coveralls

[codecov-commenter]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (c3f92dc) 82.86% compared to head (c690dd6) 82.27%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
foundationdb/src/mapped_key_values.rs	41.66%	7 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #122      +/-   ##
==========================================
- Coverage   82.86%   82.27%   -0.60%     
==========================================
  Files          23       23              
  Lines        4914     4919       +5     
==========================================
- Hits         4072     4047      -25     
- Misses        842      872      +30     

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