GIN support for @@ and @? jsonpath operators

This commit makes existing GIN operator classes jsonb_ops and json_path_ops
support "jsonb @@ jsonpath" and "jsonb @? jsonpath" operators.  Basic idea is
to extract statements of following form out of jsonpath.

 key1.key2. ... .keyN = const

The rest of jsonpath is rechecked from heap.

Catversion is bumped.

Discussion: https://postgr.es/m/fcc6fc6a-b497-f39a-923d-aa34d0c588e8%402ndQuadrant.com
Author: Nikita Glukhov, Alexander Korotkov
Reviewed-by: Jonathan Katz, Pavel Stehule

Author: akorotkov

doc/src/sgml/gin.sgml

@@ -102,13 +102,17 @@
        <literal>?&amp;</literal>
        <literal>?|</literal>
        <literal>@&gt;</literal>
+       <literal>@?</literal>
+       <literal>@@</literal>
       </entry>
      </row>
      <row>
       <entry><literal>jsonb_path_ops</literal></entry>
       <entry><type>jsonb</type></entry>
       <entry>
        <literal>@&gt;</literal>
+       <literal>@?</literal>
+       <literal>@@</literal>
       </entry>
      </row>
      <row>

doc/src/sgml/json.sgml

@@ -480,6 +480,22 @@ CREATE INDEX idxgintags ON api USING GIN ((jdoc -> 'tags'));
     (More information on expression indexes can be found in <xref
     linkend="indexes-expressional"/>.)
   </para>
+  <para>
+    Also, GIN index supports <literal>@@</literal> and <literal>@?</literal>
+    operators, which perform <literal>jsonpath</literal> matching.
+<programlisting>
+SELECT jdoc->'guid', jdoc->'name' FROM api WHERE jdoc @@ '$.tags[*] == "qui"';
+</programlisting>
+<programlisting>
+SELECT jdoc->'guid', jdoc->'name' FROM api WHERE jdoc @@ '$.tags[*] ? (@ == "qui")';
+</programlisting>
+    GIN index extracts statements of following form out of
+    <literal>jsonpath</literal>: <literal>accessors_chain = const</literal>.
+    Accessors chain may consist of <literal>.key</literal>,
+    <literal>[*]</literal> and <literal>[index]</literal> accessors.
+    <literal>jsonb_ops</literal> additionally supports <literal>.*</literal>
+    and <literal>.**</literal> statements.
+  </para>
   <para>
     Another approach to querying is to exploit containment, for example:
 <programlisting>
@@ -498,7 +514,8 @@ SELECT jdoc-&gt;'guid', jdoc-&gt;'name' FROM api WHERE jdoc @&gt; '{"tags": ["qu
 
   <para>
     Although the <literal>jsonb_path_ops</literal> operator class supports
-    only queries with the <literal>@&gt;</literal> operator, it has notable
+    only queries with the <literal>@&gt;</literal>, <literal>@@</literal>
+    and <literal>@?</literal> operators, it has notable
     performance advantages over the default operator
     class <literal>jsonb_ops</literal>.  A <literal>jsonb_path_ops</literal>
     index is usually much smaller than a <literal>jsonb_ops</literal>