reduce msrv to 1.70

[xtqqczze]

Closes #34

cc: @sylvestre

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.89%. Comparing base (3f5db7d) to head (d5e2bf5).
⚠️ Report is 20 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #58      +/-   ##
==========================================
+ Coverage   78.64%   84.89%   +6.25%     
==========================================
  Files          13       15       +2     
  Lines        2585     3410     +825     
  Branches      230      239       +9     
==========================================
+ Hits         2033     2895     +862     
+ Misses        552      515      -37     

Flag	Coverage Δ
macos_latest	84.66% <100.00%> (+7.02%)	⬆️
ubuntu_latest	84.36% <100.00%> (+6.18%)	⬆️
windows_latest	17.82% <0.00%> (+3.12%)	⬆️

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[codspeed-hq]

Merging this PR will degrade performance by 55.93%

❌ 1 regressed benchmark
✅ 28 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	num-primes	19.2 ms	43.6 ms	-55.93%

---

_{Comparing xtqqczze:msrv (1cc1e39) with main (3cf6fa1)}

[xtqqczze]

@sylvestre Are performance benchmarks stable yet?

I noticed div_ceil does produce slightly different codegen: x86_64, aarch64,

[xtqqczze]

Benches are broken for msrv 1.70 as rand_core v0.10.0 has msrv 1.85

This might be an acceptable compromise

[xtqqczze]

@sylvestre I tried to fix benches for 1.70, but couldn't fix issues with ChaCha8Rng lacking DerefMut.