[GSoC] Second period status report for the de-TOAST iterator

Hi hackers,
It's a report about my GSoC project[1] during the second period.

What I've done:
- Submitted the patch[2] to optimize partial TOAST decompression
- Submitted the patch[3] as a prototype of de-TOAST iterator and tested its
performance on position() function
- Perfected patches according to hacker's review comments

What I'm doing:
- Make overall tests on the iterator
- Applying the de-TOAST iterator to the json/jsonb data type

Thanks to hackers and mentors for helping me advance this project.

--
Best regards,
Binguo Bao

[1] https://summerofcode.withgoogle.com/projects/#6467011507388416
[2]
https://www.postgresql.org/message-id/flat/CAL-OGkthU9Gs7TZchf5OWaL-Gsi=hXqufTxKv9qpNG73d5na_g(at)mail(dot)gmail(dot)com
[3]
https://www.postgresql.org/message-id/flat/CAL-OGks_onzpc9M9bXPCztMofWULcFkyeCeKiAgXzwRL8kXiag(at)mail(dot)gmail(dot)com