Get rid of src/essential

It looks to me that [`src/essential/Essential.jl`](https://github.com/TuringLang/Turing.jl/blob/main/src/essential/Essential.jl) is no longer needed, and [`src/essential/container.jl`](https://github.com/TuringLang/Turing.jl/blob/main/src/essential/container.jl) can be merged into `src/mcmc/particle_mcmc.jl`?