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[bot]]

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

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