Remove RawArrayData

[MichalPetryka]

Removes RawArrayData and makes Array reflect the actual runtime layout instead.

Makes code a bit safer by removing Unsafe.As usages with invalid types.

cc @EgorBo @tannergooding I ended up doing this after the discussion on Discord
cc @jkotas There was one CoreCLR assert here due to array MT creation not setting the field count, I made it set it here in debug to avoid that and any perf impact in release, is that okay to do?

[MichalPetryka]

@MihuBot

[jkotas]

This should follow dotnet/runtime naming conventions for fields.

[jkotas]

@jkotas There was one CoreCLR assert here due to array MT creation not setting the field count, I made it set it here in debug to avoid that and any perf impact in release, is that okay to do?

I do not see a problem with this performance wise. However, I expect that there is going to be a long tail of places that are not prepared to handle fields on array type. Some of them may be outside dotnet/runtime. There may be more straightforward (less breaking) ways to get rid of RawArrayData. For example, call object.GetRawData() and cast it to the array layout? Or introduce JIT intrinsics for raw length and raw array data getters.

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