Remove `toJSON` implementations if/when Jest can be made to render our types

[danfuzz]

Describe the bug

The toJSON() implementations in this project only exist to help debug test cases, specifically so that expect(...).toEqual() (and similar) have a chance of rendering useful results when a test fails.

I filed Jest issue jestjs/jest#15386, which if addressed (or it turns out I missed a way to do the thing) would mean that the toJSON() methods won't be necessary anymore.

To reproduce

Steps to reproduce the problem:

1. Look for toJSON() in the code.

Expected behavior

No results.

Actual behavior

Several results.

Environment

• OS: n/a
• Node: n/a