Monadic iteration for measures?

By defining

```julia
Base.iterate(m::AbstractMeasure) = rand(m), nothing
Base.iterate(m::AbstractMeasure, ::Nothing) = rand(m), nothing
Base.Iterators.IteratorSize(::Type{<:AbstractMeasure}) = Base.IsInfinite()
```

we could make measures monadic in respect to Julia's iteration interface:

```julia
julia> using Base.Iterators, MeasureTheory
julia> take((x^2 for x in Normal()), 10) |> collect
10-element Vector{Float64}:
[...]
```

This would play well semantically with a syntax `f.(μ)` for pushforward measures (#54).

`x^2 for x in Normal` would become semantically equivalent to `x ~ Normal(); return x^2` in a model.