Enhance the query planner so that it is able to code EXISTS operators in

the WHERE clause as if they were IN operators, when appropriate.

FossilOrigin-Name: c1862abb44873f06ec0d772469d8a2d128ae4670b1e98c2d97b0e2da18df9a04

diff --cc manifest
index 1bbd748b034afb8bd332e468a222282b2f87c2bc,c51fc6703bb058f858e9a3bd0630ef55cc4f65c8..36270a6a1721733677ebf87a5c16416dbc902645
--- 1/manifest
--- 2/manifest
+++ b/manifest
@@@ -1,5 -1,5 +1,5 @@@
- C Fix\sa\shyperlink\sin\sthe\sLemon\sdocumentation.
- D 2021-01-16T12:15:41.192
 -C More\scomments\son\sthe\sEXISTS-to-IN\soptimization\slogic.
 -D 2021-01-18T00:11:20.813
++C Enhance\sthe\squery\splanner\sso\sthat\sit\sis\sable\sto\scode\sEXISTS\soperators\sin\nthe\sWHERE\sclause\sas\sif\sthey\swere\sIN\soperators,\swhen\sappropriate.
++D 2021-01-18T12:35:16.581
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  F LICENSE.md df5091916dbb40e6e9686186587125e1b2ff51f022cc334e886c19a0e9982724
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diff --cc manifest.uuid
index ffa68fe8501e8039e934fc2cb42ab1065a1e8a87,689f037d9015142059894d137273f00d827c32a7..c22949f1d497fea916e1253281bd437f0a7b150e
--- 1/manifest.uuid
--- 2/manifest.uuid
+++ b/manifest.uuid
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