feat(stable-api): Add RARRAY_AREF and RARRAY_ASET for array element access

[ianks]

Summary

Add stable API methods for accessing and setting Ruby array elements by index, equivalent to the C macros RARRAY_AREF and RARRAY_ASET.

Motivation

Direct array element access is fundamental for:

• Implementing array-like data structures
• High-performance array iteration
• Bulk array operations

The RARRAY_ASET macro is particularly important as it includes proper GC write barriers.

Ruby Source Reference

// include/ruby/internal/core/rarray.h

// Read element (currently a macro that expands to pointer access)
#define RARRAY_AREF(a, i) RARRAY_CONST_PTR(a)[i]

// Write element with GC write barrier
static inline void
RARRAY_ASET(VALUE ary, long i, VALUE v)
{
    RARRAY_PTR_USE(ary, ptr,
        RB_OBJ_WRITE(ary, &ptr[i], v));
}

Proposed API

/// Get an element from a Ruby array by index (akin to `RARRAY_AREF`).
///
/// # Safety
/// This function is unsafe because it dereferences a raw pointer and
/// does not bounds check. The caller must ensure:
/// - `obj` is a valid T_ARRAY object
/// - `idx` is within bounds (0 <= idx < RARRAY_LEN(obj))
unsafe fn rarray_aref(&self, obj: VALUE, idx: c_long) -> VALUE;

/// Set an element in a Ruby array by index (akin to `RARRAY_ASET`).
///
/// This function includes the proper GC write barrier.
///
/// # Safety
/// This function is unsafe because it:
/// - Dereferences raw pointers
/// - Modifies array contents
/// The caller must ensure:
/// - `obj` is a valid T_ARRAY object  
/// - `idx` is within bounds (0 <= idx < RARRAY_LEN(obj))
unsafe fn rarray_aset(&self, obj: VALUE, idx: c_long, val: VALUE);

Implementation Notes

• rarray_aref: Simply rarray_const_ptr(obj).offset(idx)
• rarray_aset: Must call rb_gc_writebarrier() or use RB_OBJ_WRITE pattern
• This is critical for GC correctness - cannot skip the write barrier!
• Reference: include/ruby/internal/core/rarray.h:385-403

Checklist

• Add methods to StableApiDefinition trait
• Implement for each Ruby version
• Add C fallback in compiled.c
• Add public macro wrappers in macros.rs
• Add comprehensive tests including write barrier verification
• Document GC safety requirements