Make it behave correctly when there are more than two tables being
joined, also.  Update regression test expected outputs.

  SELECT a FROM t1 tx (a);
Allow join syntax, including queries like
  SELECT * FROM t1 NATURAL JOIN t2;
Update RTE structure to hold column aliases in an Attr structure.

  * Portions Copyright (c) 1996-2000, PostgreSQL, Inc

to all files copyright Regents of Berkeley.  Man, that's a lot of files.

match then it tried for a self-commutative operator with the reversed input
data types.  This is pretty silly; there could never be such an operator,
except maybe in binary-compatible-type scenarios, and we have oper_inexact
for that.  Besides which, the oprsanity regress test would complain about
such an operator.  Remove nonfunctional code and simplify routine calling
convention accordingly.

of the SELECT part of the statement is just like a plain SELECT.  All
INSERT-specific processing happens after the SELECT parsing is done.
This eliminates many problems, e.g. INSERT ... SELECT ... GROUP BY using
the wrong column labels.  Ensure that DEFAULT clauses are coerced to
the target column type, whether or not stored clause produces the right
type.  Substantial cleanup of parser's array support.

lists are now plain old garden-variety Lists, allocated with palloc,
rather than specialized expansible-array data allocated with malloc.
This substantially simplifies their handling and eliminates several
sources of memory leakage.
Several basic types of erroneous queries (syntax error, attempt to
insert a duplicate key into a unique index) now demonstrably leak
zero bytes per query.