[SoM] Stunt coding - improve struct pinvokes, remove fixed blocks

[stephen-hawley]

For swift structs, the SwiftData member gets marshaled as an IntPtr from a fixed byte *.
Look at changing the signature of the pinvoke to instead take a byte[] and see about removing the fixed block and passing SwiftData as is.

After this work, audit manual bindings in SwiftRuntimeLibrary to adjust.