Warn users if multicore summarise_draws risks exhausting available ram

My first big test of multicore summarise_draws on a really big (~9 GB in memory) array plowed straight into the swap on my 40 GB machine and I had to kill it. Second test, after reducing cores from 4 to 3, was awesome!

If a naive user runs their big model for days and then overenthusiastically runs summarise_draws prior to saving, they could be in a world of hurt.

Given that the main use case for multicore processing with summarise_draws involves really big draws arrays, we could consider adding a .ram_safety = TRUE argument, and then doing:

if (.cores > 1 & .ram_safety & (memuse::memuse(object.size(x)) > (memuse::Sys.meminfo()$totalram / (.cores + 1)))) {
  warning(paste("The memory requirements of multi-core processing with", .cores, "cores may exceed the RAM on your system.",
                "Consider using fewer cores. If you wish to proceed with", .cores, "cores, set `.ram_safety = FALSE`",
                "and save any important objects prior to running."))
} else {
  # Do the computation
}

Main drawback is the extra package requirement for multicore (which could go in suggests).

Start at the multicore path of summarise_draws and inspect how package dependencies are declared, especially whether memuse can be added as a suggested dependency. Define the RAM warning and opt-out behavior described in the issue, then verify that multicore runs warn when estimated memory exceeds available RAM and proceed when safety is disabled.