[ANN] PackageCompiler with incremental system images

This looks really great, thanks!

I tried compiling Measurements.jl, but I get the error

 ErrorException("Task cannot be serialized")

like the one reported at https://github.com/JuliaLang/PackageCompiler.jl/issues/148. In Measurements.jl there is indeed a global constant: https://github.com/JuliaPhysics/Measurements.jl/blob/6f04d0f63bccb9d74ca680e9461dc0d41327f0d1/src/Measurements.jl#L68 But even if I change its initialization to Uint64[] PackageCompiler fails to compile it, with the same error. Is there another way to initialize the constant and make PackageCompiler happy?