This PR adds a private MiniArc type. It is 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.