[fs.walk] Explore fs.opendir

[mrmlnc]

nodejs/node#29349
nodejs/node#29941

[Fishrock123]

The perf issue should now be alleviated mostly by default, and if you want to optimize, an option for the number of buffered entries can be specified as of nodejs/node#30114, to be in the next release.

[mrmlnc]

The current implementation of opendir is twice as slow (1.8ms vs 0.8ms) for traversing the directory. This is more suitable for traversing large directories, but not for a universal tool.