Get rid of backtracking in jsonpath_scan.l

Alexander Korotkov <akorotkov@postgresql.org>

Commit: 1d88a75c424664cc85f307a876cde85191d27272
Author: Alexander Korotkov <akorotkov@postgresql.org>
Date: 2019-03-25T12:43:56Z
Releases: 12.0
Get rid of backtracking in jsonpath_scan.l

Non-backtracking flex parsers work faster than backtracking ones.  So, this
commit gets rid of backtracking in jsonpath_scan.l.  That required explicit
handling of some cases as well as manual backtracking for some cases.  More
regression tests for numerics are added.

Discussion: https://mail.google.com/mail/u/0?ik=a20b091faa&view=om&permmsgid=msg-f%3A1628425344167939063
Author: John Naylor, Nikita Gluknov, Alexander Korotkov

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