Read up to 32 directory entries in one batch when `dir.readSync()`
or `dir.read()` are called.

This increases performance significantly, although it introduces
quite a bit of edge case complexity.

                                                                 confidence improvement accuracy (*)    (**)    (***)
     fs/bench-opendir.js mode='async' dir='lib' n=100                  ***    155.93 %      ±30.05% ±40.34%  ±53.21%
     fs/bench-opendir.js mode='async' dir='test/parallel' n=100        ***    479.65 %      ±56.81% ±76.47% ±101.32%
     fs/bench-opendir.js mode='sync' dir='lib' n=100                           10.38 %      ±14.39% ±19.16%  ±24.96%
     fs/bench-opendir.js mode='sync' dir='test/parallel' n=100         ***     63.13 %      ±12.84% ±17.18%  ±22.58%