Prepare for custom RNGs in future package

[rstub]

After a great initial discussion with @HenrikBengtsson I should prepare for the possibility of custom RNGs in the future package. In particular:

• Benchmark switching streams with various RNGs, since this is one of the bottlenecks for R's parallel RNG ("L'Ecuyer-CMRG")
• How can both streams and substreams be provided for the different RNGs?
• How to detect/prevent overlap between streams, especially with nested parallelization?
• Is the existing method to get/set the raw state of the RNG sufficient?

[HenrikBengtsson]

FYI, I've prepared an API for this in the future package (that's on CRAN since 2025-04-10). It's currently a draft, internal API, and it encapsulates the L'Ecuyer-CMRG RNG API by default. I've made future to use this API internally instead of call L'Ecuyer-CMRG RNG functions explicitly. This opens up the door for using other RNGs such as yours.

Assuming it wasn't already set, here's how you would make L'Ecuyer-CMRG the default RNG in Futureverse:

future:::parallel_rng_kind(
              kind = "L'Ecuyer-CMRG",
          set_kind = base::RNGkind,
       next_stream = parallel::nextRNGStream,
    next_substream = parallel::nextRNGSubStream,
           is_seed = future:::is_lecyer_cmrg_seed,
           as_seed = future:::as_lecyer_cmrg_seed
)

[rstub]

Thanks @HenrikBengtsson!