Optimize fast_locals and atomic ordering by youknowone · Pull Request #7289 · RustPython/RustPython

- inc/inc_by/get: SeqCst → Relaxed
- safe_inc CAS: SeqCst → Relaxed + compare_exchange_weak
- dec: SeqCst → Release + Acquire fence when count drops to 0
- leak CAS: SeqCst → AcqRel/Relaxed + compare_exchange_weak

Replace vec![self_val] + extend(args.args) with
FuncArgs::prepend_arg() to avoid a second heap allocation
on every method call.

Early return in PyCallable::invoke() when use_tracing is false,
avoiding two downcast_ref type checks on every function call.

* Relax RefCount atomic ordering from SeqCst to Arc pattern

- inc/inc_by/get: SeqCst → Relaxed
- safe_inc CAS: SeqCst → Relaxed + compare_exchange_weak
- dec: SeqCst → Release + Acquire fence when count drops to 0
- leak CAS: SeqCst → AcqRel/Relaxed + compare_exchange_weak

* Reuse existing Vec via prepend_arg in execute_call

Replace vec![self_val] + extend(args.args) with
FuncArgs::prepend_arg() to avoid a second heap allocation
on every method call.

* Skip downcast_ref checks in invoke when tracing is disabled

Early return in PyCallable::invoke() when use_tracing is false,
avoiding two downcast_ref type checks on every function call.

* Replace fastlocals PyMutex with UnsafeCell-based FastLocals

Eliminate per-instruction mutex lock/unlock overhead for local
variable access. FastLocals uses UnsafeCell with safety guaranteed
by the frame's state mutex and sequential same-thread execution.

Affects 14+ lock() call sites in hot instruction paths (LoadFast,
StoreFast, DeleteFast, and their paired variants).

* Auto-format: cargo fmt --all

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