Consider Reworking Platform Props

[andreiborodin]

Today, we generate platform props per-platform using AssemblyBuilder, in 2019 it may be possible to significantly reduce this with:
CompilationPipeline.GetPrecompiledAssemblyPaths(CompilationPipeline.PrecompiledAssemblySources.UserAssembly)
CompilationPipeline.GetPrecompiledAssemblyPaths(CompilationPipeline.PrecompiledAssemblySources.SystemAssembly)
CompilationPipeline.GetPrecompiledAssemblyPaths(CompilationPipeline.PrecompiledAssemblySources.UnityEditor)

Furthermore, we may just pre-generate for a specific Unity version.