Functional uncertainty

[apfeffer]

One of the typical patterns in functional probabilistic programming is to apply a function whose identity is uncertain to arguments that are also uncertainty. Figaro's monadic Chain and Apply framework don't make it easy to do this. We should add a class of element over functions, and define an apply method on it, so you can write

f(x)

where f and x are both elements.

[motownblue]

also work where f is an element and x is value