Add pinning support for BigArray

Hezium.Memory.Tests/CoreApiTests.cs

@@ -93,6 +93,23 @@ public void BigArray_ExtensionApis_Work()
         array2.Fill(null);
         array2[3] = 42;
         Assert.Equal([null, null, null, 42, null, null, null], array2.ToArray());
+
+        BigArray<int> bigArray = GC.AllocateBigArray<int>(8, pinned: true);
+        for (int i = 0; i < bigArray.Length; ++i)
+        {
+            bigArray[i] = i * 2;
+        }
+
+        unsafe
+        {
+            fixed (int* ttPtr = bigArray)
+            {
+                for (int i = 0; i < bigArray.Length; ++i)
+                {
+                    Assert.Equal(bigArray[i], *(ttPtr + i));
+                }
+            }
+        }
     }
 
     [Fact]

Hezium.Memory/BigArray.cs

@@ -210,4 +210,19 @@ public Span<T> AsSpan(nint start, int length)
 
         return MemoryMarshal.CreateSpan(ref Unsafe.Add(ref MemoryExtensions.GetBigArrayDataReference(this), start), length);
     }
+
+    /// <summary>
+    /// Gets a reference to the element that can be used for pinning.
+    /// </summary>
+    /// <returns>A reference to the first element of the span, or a null reference if the array is empty.</returns>
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public ref T GetPinnableReference()
+    {
+        if (_length == 0)
+        {
+            return ref Unsafe.NullRef<T>();
+        }
+
+        return ref MemoryExtensions.GetBigArrayDataReference(this);
+    }
 }