Upgrading to a truly AnyCPU assembly

[aybe]

Preliminary generation done here:

https://github.com/aybe/imgui.NET/tree/AnyCPU

Things that differ between architectures:

size_t, StructLayout.Size, DllImport.EntryPoint, DllImport.CallingConvention

Things to look out for:

• transparent loading and merging of these inner assemblies

[aybe]

74a44d5eac6076d0297b2b81ec6390b22a738b01