The main optimizations are
- Removed visibleKeys
- Removed proxy cache
- Removed Object.assign
- No key concatenating anymore
- No key recalculating anymore
- Improved indentation logic
- Improved string escape logic
- Added many fast paths
- Optimized code branches a lot
- Optimized (boxed) primitive handling
- Inline code if possible
- Only check extra keys if necessary
- Guard against unnecessary more expensive calls

This also fixes a bug with special array number keys as e.g. "00".

Besides that there were lots of smaller optimizations, the
code got a bit cleaned up and a few more tests got in.

PR-URL: nodejs#14881
Fixes: nodejs#15288
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

This fixes a issue brought up in nodejs#15288.

PR-URL: nodejs#14881
Refs: nodejs#15288
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

Remove unnecessary code parts and outdated code

PR-URL: nodejs#14881
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>