WIP: Ractor harness

[casperisfine]

Some of the simpler benchmarks should be relatively easy to run in ractors, and should be mostly perfectly parallelisable, so it could be interesting to try to run them concurrently in a handful of ractors to see if they perform about as well in these conditions.

So far I only tried to run the binarytree benchmark, and it seems it's contending on something (GC?)

$ ruby --yjit benchmark.rb 
ruby 3.5.0dev (2025-07-23T11:23:38Z master 85221800ca) +YJIT +PRISM [arm64-darwin24]
itr:   time
 #1:  115ms
 #2:  110ms
 #3:  102ms
 #4:  105ms
...
$ RACTOR_PARALLEL=4 ruby --yjit benchmark.rb 
ruby 3.5.0dev (2025-07-23T11:23:38Z master 85221800ca) +YJIT +PRISM [arm64-darwin24]
itr:   time
 #1:  335ms
 #1:  340ms
 #1:  346ms
 #1:  362ms
 #2:  292ms
 #2:  314ms