This PR fixes an issue with `explore` where you can "drill down" into
the same value forever. For example:
1. Run `ls | explore`
2. Press Enter to enter cursor mode
3. Press Enter again to open the selected string in a new layer
4. Press Enter again to open that string in a new layer
5. Press Enter again to open that string in a new layer
6. Repeat and eventually you have a bajillion layers open with the same
string
IMO it only makes sense to "drill down" into lists and records.
In a separate commit I also did a little refactoring, cleaning up naming
and comments.
8707d14f95