- Communicate the current async stack length through a
  typed array field rather than a native binding method
- Add a new fixed-size `async_ids_fast_stack` typed array
  that contains the async ID stack up to a fixed limit.
  This increases performance noticeably, since most of the time
  the async ID stack will not be more than a handful of
  levels deep.
- Make the JS `pushAsyncIds()` and `popAsyncIds()` functions
  do the same thing as the native ones if the fast path
  is applicable.

Benchmarks:

    $ ./node benchmark/compare.js --new ./node --old ./node-master --runs 10 --filter next-tick process | Rscript benchmark/compare.R
    [00:03:25|% 100| 6/6 files | 20/20 runs | 1/1 configs]: Done
                                                   improvement confidence      p.value
     process/next-tick-breadth-args.js millions=4     19.72 %        *** 3.013913e-06
     process/next-tick-breadth.js millions=4          27.33 %        *** 5.847983e-11
     process/next-tick-depth-args.js millions=12      40.08 %        *** 1.237127e-13
     process/next-tick-depth.js millions=12           77.27 %        *** 1.413290e-11
     process/next-tick-exec-args.js millions=5        13.58 %        *** 1.245180e-07
     process/next-tick-exec.js millions=5             16.80 %        *** 2.961386e-07