Replace Marshal with Unsafe

[andreakarasho]

No description provided.

[BasGeertsema]

I noticed the warning in the documentation. It seems to me that we actually want the size of the unmanaged view of the type? In that case we would need to use Marshal.SizeOf, as it is now.

[andreakarasho]

Partially reverted to the Marshalversion but keeping the constraint version.

[jsturtevant]

Should we try using the internal structs version before updating this? #332 (comment)

I think using the internal structs would remove the need for this change

[BasGeertsema]

Should we try using the internal structs version before updating this? #332 (comment)

I think using the internal structs would remove the need for this change

I agree, that would be a better solution.