SE-0260: Library Evolution for Stable ABIs

There's precedent for this in other languages which IMO would map fairly naturally to Swift. For example, C# has the [StructLayout(LayoutKind.Sequential)] attribute, which could map to a @structLayout(sequential) attribute. Of course, this is largely a moot issue until the compiler's layout algorithm changes.

By the way, you currently can't map trivially to GPU structs in many cases because the struct's size isn't rounded up to the maximum alignment of the stored properties. Assuming a struct of the below format with Vec3 being an alignment-16 type,

the struct definition needs an extra let padding : Vec2 = Vec2(0) property at the end to make it compatible with GPU structs under the current layout algorithm.