Use `@rand` or `@sample` instead of `rand`?

[gbdrt]

Currently calls to rand are capture to execute custom code via internal_rand(e.g., add a node for symbolic computation). Why not use a macro, e.g., @rand or @sample (similar to @observe)

That would also be easier to document, since right now there is no function rand in OnlineSampling that can be described in the doc.