fs: refactor to use private fields

[Lxxyx]

Current thoughts are personal, feel free to point out problems if inappropriate!

Refactor lib/internal/fs/dir.js and lib/internal/fs/util.js to use private fields instead of symbols.

Reason:

• Symbol used to emulate Private Fields (Using global symbols), but now Private Fields are available (proposal-class-fields#Private fields).
• Symbol makes the code not easy to understand. We need to jump around between uses and definitions.
• Symbols are used in Node.js in two ways.
  1. to implement private variables only.
  2. for variables that may be used by internal code, but do not want to be modified externally.

When I see a Symbol, I need to look at the context to determine if the variable is just private or shared by internal. I think Symbol only do option 2 in the project, which will make the code easier to read.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

[mscdex]

One of the reasons for not using private fields yet is performance. They're still not as fast as normal properties or even Symbols:

                                                                              confidence improvement accuracy (*)    (**)   (***)
fs/bench-opendir.js bufferSize=1024 mode='callback' dir='test/parallel' n=100        ***    -11.08 %       ±3.95%  ±5.26%  ±6.86%
fs/bench-opendir.js bufferSize=1024 mode='sync' dir='test/parallel' n=100            ***    -11.78 %       ±4.02%  ±5.41%  ±7.16%
fs/bench-opendir.js bufferSize=32 mode='callback' dir='test/parallel' n=100          ***    -11.43 %       ±5.15%  ±6.86%  ±8.93%
fs/bench-opendir.js bufferSize=32 mode='sync' dir='test/parallel' n=100              ***    -15.40 %       ±0.97%  ±1.29%  ±1.68%
fs/bench-opendir.js bufferSize=4 mode='callback' dir='test/parallel' n=100           ***     -6.42 %       ±3.65%  ±4.86%  ±6.33%
fs/bench-opendir.js bufferSize=4 mode='sync' dir='test/parallel' n=100               ***    -14.61 %       ±1.09%  ±1.45%  ±1.90%

                                                                       confidence improvement accuracy (*)   (**)   (***)
fs/bench-readdir.js withFileTypes='true' dir='test/parallel' n=10             ***    -12.21 %       ±2.64% ±3.52%  ±4.58%
fs/bench-readdirSync.js withFileTypes='true' dir='lib' n=10                   ***    -13.28 %       ±5.36% ±7.14%  ±9.33%
fs/bench-readdirSync.js withFileTypes='true' dir='test/parallel' n=10         ***     -9.81 %       ±3.70% ±4.92%  ±6.40%

[Lxxyx]

One of the reasons for not using private fields yet is performance. They're still not as fast as normal properties or even Symbols:

                                                                              confidence improvement accuracy (*)    (**)   (***)
fs/bench-opendir.js bufferSize=1024 mode='callback' dir='test/parallel' n=100        ***    -11.08 %       ±3.95%  ±5.26%  ±6.86%
fs/bench-opendir.js bufferSize=1024 mode='sync' dir='test/parallel' n=100            ***    -11.78 %       ±4.02%  ±5.41%  ±7.16%
fs/bench-opendir.js bufferSize=32 mode='callback' dir='test/parallel' n=100          ***    -11.43 %       ±5.15%  ±6.86%  ±8.93%
fs/bench-opendir.js bufferSize=32 mode='sync' dir='test/parallel' n=100              ***    -15.40 %       ±0.97%  ±1.29%  ±1.68%
fs/bench-opendir.js bufferSize=4 mode='callback' dir='test/parallel' n=100           ***     -6.42 %       ±3.65%  ±4.86%  ±6.33%
fs/bench-opendir.js bufferSize=4 mode='sync' dir='test/parallel' n=100               ***    -14.61 %       ±1.09%  ±1.45%  ±1.90%

                                                                       confidence improvement accuracy (*)   (**)   (***)
fs/bench-readdir.js withFileTypes='true' dir='test/parallel' n=10             ***    -12.21 %       ±2.64% ±3.52%  ±4.58%
fs/bench-readdirSync.js withFileTypes='true' dir='lib' n=10                   ***    -13.28 %       ±5.36% ±7.14%  ±9.33%
fs/bench-readdirSync.js withFileTypes='true' dir='test/parallel' n=10         ***     -9.81 %       ±3.70% ±4.92%  ±6.40%

Thanks for the reply, I hadn't noticed the performance issue before, maybe we need to wait for the improvement from V8.