Bump rust to 1.76 and disable sleeping between kernels on Win

[poszu]

Fixes #209

Two changes aim to improve the situation on Windows:

• bumped Rust to 1.76. std::thread::sleep() was improved to have submillisecond resolution (see Windows: Support sub-millisecond sleep rust-lang/rust#116461 & https://github.com/rust-lang/rust/releases/tag/1.75.0) in Rust 1.75,
• changed logic calculating the sleep time:
  • treat the first 10 iterations as a warmup to allow the GPU to gain some speed before starting to measure kernel execution times,
  • (windows only) the maximum sleeping time is average - 5ms to give the scheduler more time to continue the initialization thread.

[codecov]

Codecov Report

Attention: Patch coverage is 44.00000% with 14 lines in your changes are missing coverage. Please review.

Project coverage is 93.92%. Comparing base (1460e07) to head (2468cd5).
Report is 8 commits behind head on main.

Files	Patch %	Lines
scrypt-ocl/src/lib.rs	44.00%	14 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #210      +/-   ##
==========================================
- Coverage   94.28%   93.92%   -0.37%     
==========================================
  Files          26       26              
  Lines        3326     3374      +48     
==========================================
+ Hits         3136     3169      +33     
- Misses        190      205      +15     

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