The benchmark shows that HashCode is faster than murmurhash3:

```
|     Method |     N |       Mean |   Error |  StdDev |
|----------- |------ |-----------:|--------:|--------:|
| MurmurHash |  1000 |   209.1 ns | 0.11 ns | 0.11 ns |
|   HashCode |  1000 |   133.8 ns | 0.18 ns | 0.17 ns |
| MurmurHash | 10000 | 2,117.4 ns | 2.74 ns | 2.43 ns |
|   HashCode | 10000 | 1,349.3 ns | 2.95 ns | 2.62 ns |
```

Based on this removed MurmurHash3.cs entirely and replaced all uses with `HashCode`.

src/Arch/Core/Extensions/Internal/HashCodeExtensions.cs

@@ -0,0 +1,26 @@
+namespace Arch.Core.Extensions.Internal;
+
+internal static class HashCodeExtensions
+{
+#if !NET5_0_OR_GREATER
+    public static void AddBytes(ref this HashCode hashCode, ReadOnlySpan<byte> bytes)
+    {
+        for (int i = 0; i < bytes.Length; i++)
+        {
+            hashCode.Add(bytes[i]);
+        }
+    }
+#endif
+
+    public static void AddSpan<T>(ref this HashCode hashCode, ReadOnlySpan<T> items)
+        where T : unmanaged
+    {
+        hashCode.AddBytes(MemoryMarshal.AsBytes(items));
+    }
+
+    public static void AddSpan<T>(ref this HashCode hashCode, Span<T> items)
+        where T : unmanaged
+    {
+        hashCode.AddBytes(MemoryMarshal.AsBytes(items));
+    }
+}

src/Arch/Core/Utils/CompileTimeStatics.cs

@@ -455,17 +455,12 @@ public static int GetHashCode(Span<ComponentType> obj)
           return GetHashCode(stack);
     }
 
-    /// <summary>
-    ///     Calculates the hash code of a bitset span, which is unique for the elements contained in the array.
-    ///     The order of the elements does not change the hashcode, so it depends on the elements themselves.
-    /// </summary>
-    /// <param name="obj">The <see cref="BitSet"/>.</param>
-    /// <returns>A unique hashcode for the contained elements, regardless of their order.</returns>
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public static int GetHashCode(Span<uint> span)
     {
-        var bytes = MemoryMarshal.AsBytes(span);
-        return (int)MurmurHash3.Hash32(bytes, 0);
+        var hashCode = new HashCode();
+        hashCode.AddSpan(span);
+        return hashCode.ToHashCode();
     }
 }