`new` syntax is only possible with indexed statements

[wouterwln]

I want to be able to do this:

@model function submodel(x, y, z)
    z ~ Gamma(1, 1)
    x ~ NormalMeanPrecision(y, z)
end

@model function some_model(y)
    x ~ submodel(y=y, z=new(z))
end