Add a benchmark of atomic increments

[polytypic]

This adds a benchmark of incrementing a int Atomic.t using Atomic.incr and Atomic.compare_and_set with and without randomized exponential backoff.

This benchmark is highly CPU dependent. Here are results from a run my M3 Max with OCaml 5.2:

➜  multicore-bench git:(add-incr-bench) dune exec --release -- bench/main.exe -budget 1 -brief incr
Atomic incr:                         
  time per op/CAS, 1 domains:
    4.89 ns
  ops over time/CAS, 1 domains:
    204.47 M/s
  time per op/CAS, 2 domains:
    34.43 ns
  ops over time/CAS, 2 domains:
    58.09 M/s
  time per op/CAS, 4 domains:
    160.78 ns
  ops over time/CAS, 4 domains:
    24.88 M/s
  time per op/CAS, 8 domains:
    1267.91 ns
  ops over time/CAS, 8 domains:
    6.33 M/s
  time per op/CAS with backoff, 1 domains:
    4.86 ns
  ops over time/CAS with backoff, 1 domains:
    205.77 M/s
  time per op/CAS with backoff, 2 domains:
    10.09 ns
  ops over time/CAS with backoff, 2 domains:
    198.26 M/s
  time per op/CAS with backoff, 4 domains:
    22.73 ns
  ops over time/CAS with backoff, 4 domains:
    176.00 M/s
  time per op/CAS with backoff, 8 domains:
    55.53 ns
  ops over time/CAS with backoff, 8 domains:
    144.06 M/s
  time per op/Incr, 1 domains:
    1.90 ns
  ops over time/Incr, 1 domains:
    527.59 M/s
  time per op/Incr, 2 domains:
    11.38 ns
  ops over time/Incr, 2 domains:
    175.74 M/s
  time per op/Incr, 4 domains:
    40.06 ns
  ops over time/Incr, 4 domains:
    99.84 M/s
  time per op/Incr, 8 domains:
    349.91 ns
  ops over time/Incr, 8 domains:
    22.86 M/s

It is particularly interesting that Atomic.incr seems to suffer more from contentention on M3 Max with OCaml 5.2 than CAS with backoff.