Re: Extract numeric filed in JSONB more effectively

Here is latest version, nothing changed besides the rebase to the latest
master. The most recent 3 questions should be addressed.

- The error message compatible issue [1] and the Peter's answer at [2].
- Peter's new question at [2] and my answer at [3].

Any effrot to move this patch ahead is welcome and thanks all the people
who provided high quaility feedback so far, especially chapman!

[1] https://www.postgresql.org/message-id/87r0hmvuvr.fsf@163.com
[2]
https://www.postgresql.org/message-id/8102ff5b-b156-409e-a48f-e53e63a39b36%40eisentraut.org
[3] https://www.postgresql.org/message-id/8734t6c5rh.fsf%40163.com

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Best Regards
Andy Fan