Support random variables that use `rand`

[zenna]

Most existing Julia samplers that use rand(rng)

For instance

function X(rng)
  a = rand(rng)
  b = rand(rng)
  a + b
end

This should be equivalent to something like

function X2(rng)
  a = (1 ~ Uniform(0, 1))(rng)
  b = (2 ~ Uniform(0, 1))(rng)
  a + b
end

One issue is that this is not a transformation that can be done locally in rand using method dispatch, since a and b need different ids.

Before I was doing this using a counter, resetting the counter at the start of the random variable. The problem is the notion of the "start" of a random variable is somewhat unclear, especially since OmegaCore does not focus so much on the RandVar type.

For example, suppose we have

function X3(rng)
  a = rand(rng)
  b = rand(rng)
  c = rand(rng)
  d = rand(rng)
  a + b + c + d 
end

We could rewrite this as

function helper(rng)
  x = rand(rng)
  y = rand(rng)
  x + y
end

function X4(rng)
  helper(rng) + helper(rng)
end

So here helper doesn't reset but X4 does?

We need some way to distinguish between thees things: If I had control over the syntax then I can say X4 should be:

function X4b(rng)
  ~ helper(rng) + ~ helper(rng)
end

Perhaps the first thing is to figure out the correct syntax when I have control and translate rand into that.