Switch from triomphe::Arc to MiniArc, our own Arc implementation

[tatsuya6502]

This PR adds a crate private MiniArc type, a minimal implementation of atomic reference counted shared pointer. It is like a std::sync::Arc and triomphe::Arc but with a few differences:

• No Weak references, so MiniArc does not have the weak reference counter.
• Uses AtomicU32 instead of AtomicUsize for the strong reference counter.
• Much smaller code size (~100 lines) by having only the necessary methods for moka and mini-moka.

MiniArc replaces all triomphe::Arc usages in moka.

On the CI (GitHub Actions), the following tests are performed in addition to the regular tests:

• Miri tests on MiniArc.
• Loom tests on MiniArc.

---

Fixes #440

[codecov]

Codecov Report

Attention: Patch coverage is 90.61033% with 20 lines in your changes missing coverage. Please review.

Project coverage is 94.73%. Comparing base (bd5e447) to head (14651ee).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #456      +/-   ##
==========================================
- Coverage   94.79%   94.73%   -0.07%     
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  Files          44       45       +1     
  Lines       20306    20416     +110     
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+ Hits        19249    19341      +92     
- Misses       1057     1075      +18