Add broadcasting `tilde` operation

[bvdmitri]

Would be nice to support broadcasting operation for tilde operation in the following way:

@model function mymodel(c)
     y = datavar(Float64, n)
     x = randomvar(n)
     
     x .~ Normal(0.0, 1.0) # iid, unrolls to for array automatically
     y .~ Normal(x, 1.0) # equivalent to 'for each y_i ~ Normal(x_i, 1.0)`
end

It should be relatively easy to implement with the built-in broadcast function.

Creating random variables from broadcasting operation with AutoVar might be challenging, not impossible though.

@model function mymodel(c)
     y = datavar(Float64, n)
     x = randomvar(n)
     
     x .~ Normal(0.0, 1.0) # iid, unrolls to for array automatically
     z .~ Normal(x, 1.0) #  z has not been defined before, but might be created automatically
     y .~ Normal(x, 1.0) # equivalent to 'for each y_i ~ Normal(x_i, 1.0)`
end

[bvdmitri]

This would also allow for some cool stuff like:

@model function state_space_model(n)
    x = randomvar(n)
    y = datavar(Float64, n)
    x[1] ~ Normal(0.0, 1.0)
    x[2:end] .~ Normal(A .* x[1:end-1], P)
    y .~ Normal(x, 1.0)
end

[bvdmitri]

Added in #14