Don't use jl_value_ptr

[simonbyrne]

Wrap immutable objects in a Ref to avoid the jl_value_ptr problems

[randy3k]

Thanks

[simonbyrne]

I'm not 100% sure how necessary this is, but probably better to be on the safe side.

[simonbyrne]

@yuyichao Is this handling of function callbacks correct? Or is there a better way we should be doing this?

[yuyichao]

Looks correct. A better way, if the performance of the callback is important, would be to do sth similar to JuliaMath/Cubature.jl#23 to allow specialization of the callback.