WL#8607: Inline JSON path expressions in SQL

This WL adds inlining JSON path expressions in SQL queries to MySQL server. It
should allow to execute queries like:

 SELECT ... FROM t1 WHERE t1.json_field->"$.path.to[0].key"= 123;

here 
  't1.json_field' is the column where specified path is searched,
  "$.path.to[0].key" is the JSON path expression described in WL#7909,
  '->' is used as separator between column and string literal containing path.

Essentially, this is a syntactic sugar and the query above equivalent to
following:

 SELECT ... FROM t1 WHERE JSN_EXTRACT(t1.json_field, "path.to[0].key") = 123;

This WL will do this translation during parsing.

Functional requirements
-----------------------
1) Inlined JSON path expressions should be allowed to appear anywhere a
simple expression may appear. Similar applied to where json_* funcs aren't 
allowed.
2) A query with inlined JSON path expr should produce exactly same result and
execution plan as with corresponding json_extract function.

Non-functional requirements
---------------------------
No bugs/regressions should be introduced.

Outline
-------
This WL aims to allow to inline JSON path expressions into SQL. For example:

SELECT f1->"$.a" FROM t1 WHERE f1->"$.b" = 123 ORDER BY f1->"$.datetime";

here:
  f1       is a column identifier, general expressions aren't supported
  ->       is a separator, could be surrounded by spaces (i.e "f1 -> '$.a'"
           is valid expression)
  "" is a string literal containing path expression that's acceptable by
           JSON_EXTRACT function (see WL#7909). Nothing but string literal is
           accepted here. Both ' (single quote) and " (double quote) could be
           used to surround string.


It's implemented as pure syntactic sugar and completely resides in parser and
lexer.  The query above is equivalent to following:

SELECT json_extract(f1, "$.a") FROM t1
  WHERE json_extract(f1, "$.b") = 123 ORDER BY json_extract(f1, "$.datetime");

Due to syntactic sugar nature, all errors, warnings, etc are reported under
name of JSON_EXTRACT function. EXPLAIN will print a call to JSON_EXTRACT which
would have inlined path expression as alias, e.g
  f1->"$.a"
will be printed as
  json_extract(`test`.`t1`.`f1`,'$.a') AS `f1->"$.a"`

The inlined path expression should be allowed everywhere, where regular call
to JSON_EXTRACT function is allowed. For example (but not limited to):
select list, WHERE/HAVING clauses, ORDER/GROUP BY clauses, etc.

Note on separator
----------------------
As agreed at the optimizer team meeting'15 we'll use '->' as a separator, as
it causes least amount of disagreement.

Changes to parser
-----------------
A rule in parser is extended to create json_extract() function when JSON path
expression is found.

simple_expr:
        ...
        | simple_ident JSON_SEPARATOR_SYM json_path_expr
          {
            // create function: json_extract(,)
          }

Changes to lexer 
----------------
Lexer now can additionally return one new token: JSON_SEPARATOR_SYM.
It denotes '->'. This is needed to avoid introducing shift/reduce conflicts in
parser.