Add IntPtr property accessors for structs with native pointers

[AArnott]

This work represents both the changes to remove the IntPtr fields from existing structs, replacing them with native pointer types, as well as adding the IntPtr property accessors for those new fields.

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[AArnott]

I've confirmed that VB has no problem using structs that contain native pointer fields. It just hides them from Intellisense, allowing access to the non-pointer fields and properties without problem. So the IntPtr property accessor approach should work fine.