In this PR we improve the performance of babel-traverse by revising the memory layout of NodePath. It results to 10% performance gain compared to the control branch on the following test cases.

const babel = require("/path/to/local/babel-core");
const code = require("fs").readFileSync("./fixtures/es5/babylon-dist.js");
babel.transform(code, { preset: ["@babel/env"] });

I can observe this branch outperforms control branch consistently.

┌─────────────────────┬──────────────────────────────┐
│ fixture             │ devTransform                 │
├─────────────────────┼──────────────────────────────┤
│ es5/babylon-dist.js │ 1.02 ops/sec ±31.84% (978ms) │
└─────────────────────┴──────────────────────────────┘
┌─────────────────────┬───────────────────────────────┐
│ fixture             │ baseTransform                 │
├─────────────────────┼───────────────────────────────┤
│ es5/babylon-dist.js │ 0.86 ops/sec ±20.52% (1167ms) │
└─────────────────────┴───────────────────────────────┘

And the memory usage improvements due to compressing traverse flags and adding accessor methods for inList and parentPath.

┌─────────────────────┬──────────────────┐
│ fixture             │ devTransform x5  │
├─────────────────────┼──────────────────┤
│ es5/babylon-dist.js │ heap: 239.37 MiB │
└─────────────────────┴──────────────────┘
┌─────────────────────┬──────────────────┐
│ fixture             │ baseTransform x5 │
├─────────────────────┼──────────────────┤
│ es5/babylon-dist.js │ heap: 305.47 MiB │
└─────────────────────┴──────────────────┘

The following changes are implemented:

• Removed NodePath.inList data property, a get accessor is provided for backward compatibility (partially).
• Removed NodePath.parentKey data property, a get accessor is provided for backward compatibility (partially).
• Removed NodePath.typeAnnotation data property, it is introduced in Initialize properties to avoid hidden class thrashing. #1752 but since then it is not used by any routines. I believe it is a mistype of Node.typeAnnotation.
• Compressed NodePath.shouldSkip/shouldStop/removed into _traverseFlags bit array, both get accessor and set accessor is provided for backward compatibility.
• Lazily assign this.skipKeys in setContext.
• Lazily initialize this.data in NodePath.constructor.

In practice inList and parentKey should be treated as read-only since these two properties are rewritten in setContext. The current behavior accepts one assigning these two properties.

The shouldSkip/shouldStop/removed traverse flags are handy and turbofan would inline these simple accessors we don't introduce performance overhead here. By compressing these states into bit array we optimize the performance of setContext.