Parallelizing loo

The `loo` package is a widely used R package in the Stan ecosystem which exposes a clean API to cross-validate (CV) Bayesian models. Currently, some `loo` functions use parallelization by way of forking processes, which doesn't work on Windows but has some benefits over other parallelization schemes with respect to memory. The `mirai` package is a new approach to cross-platform parallelization which relies on daemons, allowing package developers to write parallel code without any knowledge of where the code will be executed--allowing users to scale up to any number of local or remote cores. `mirai` represents one approach to updating `loo`'s approach to parallelization. After this project, users of `loo` will be able to run some expensive calls in parallel, regardless of the OS they use, and ideally in any environment they would like--be it a SLURM cluster, SSH access to a large server, across a homelab, or just on many cores on their local machine.