Allow named parameters to be specified using => in addition to :=

SQL has standardized on => as the use of to specify named parameters,
and we've wanted for many years to support the same syntax ourselves,
but this has been complicated by the possible use of => as an operator
name.  In PostgreSQL 9.0, we began emitting a warning when an operator
named => was defined, and in PostgreSQL 9.2, we stopped shipping a
=>(text, text) operator as part of hstore.  By the time the next major
version of PostgreSQL is released, => will have been deprecated for a
full five years, so hopefully there won't be too many people still
relying on it.  We continue to support := for compatibility with
previous PostgreSQL releases.

Pavel Stehule, reviewed by Petr Jelinek, with a few documentation
tweaks by me.

src/test/regress/sql/polymorphism.sql

@@ -748,6 +748,22 @@ select dfunc('a'::text, 'b', flag := false); -- mixed notation
 select dfunc('a'::text, 'b', true); -- full positional notation
 select dfunc('a'::text, 'b', flag := true); -- mixed notation
 
+-- ansi/sql syntax
+select dfunc(a => 1, b => 2);
+select dfunc(a => 'a'::text, b => 'b');
+select dfunc(a => 'a'::text, b => 'b', flag => false); -- named notation
+
+select dfunc(b => 'b'::text, a => 'a'); -- named notation with default
+select dfunc(a => 'a'::text, flag => true); -- named notation with default
+select dfunc(a => 'a'::text, flag => false); -- named notation with default
+select dfunc(b => 'b'::text, a => 'a', flag => true); -- named notation
+
+select dfunc('a'::text, 'b', false); -- full positional notation
+select dfunc('a'::text, 'b', flag => false); -- mixed notation
+select dfunc('a'::text, 'b', true); -- full positional notation
+select dfunc('a'::text, 'b', flag => true); -- mixed notation
+
+
 -- check reverse-listing of named-arg calls
 CREATE VIEW dfview AS
    SELECT q1, q2,