Support sampling from model in model

[xukai92]

We should later support something like this.

@model a(...) = begin
    # code
end

@model b(...) = begin
    x ~ a(...)
    # other code
end

[yebai]

This is a really nice feature. I have been thinking about this; it may require some substantial work on the internals. Let's spend sometime think about how this might be designed.

[xukai92]

I guess the internal are good enough for us the work on this now?

[yebai]

Fixed by #233