Add `feature/native` - An arch variant only using native memory

[genaray]

Problem

Arch itself uses managed arrays. There's no way around it, this way it can support managed and unmanaged components.
However, not anyone needs managed arrays since they create additional GC pressure.

The hybrid feature/unsafe variant exists to combine unmanaged and managed memory efficiently. However, it is rarely used and is a bit slower than the master. Unfortunately feature/unsafe can not provide a clean way to split between native and managed arrays and therefore it causes a lot of branching during queries or lookups (since it is always forced to check whether it's a managed or unmanaged array that is being accessed: https://github.com/genaray/Arch/blob/feature/unsafe/src/Arch/Core/Chunk.cs#L144 ). I haven't found a clean way yet to separate those and prevent the branching.

Solution

It might be easier and more efficient to create a feature/native branch instead that ONLY uses native memory for those arrays. Classes and references would be forced to lay outside the ECS, but it would be a good fast alternative to the master for those wanting the maximum performance. feature/unsafe would be obsolete and discontinued in favor of feature/native.

[martindevans]

See Discord discussion here: https://discord.com/channels/1099813114876284928/1100418566706171974/1171139303288602735

Summary: if (RuntimeHelpers.IsReferenceOrContainsReferences<T>()) is JIT intrinsic and is free to branch on.