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/backend/parser/scan.l

@@ -334,6 +334,7 @@ identifier		{ident_start}{ident_cont}*
 typecast		"::"
 dot_dot			\.\.
 colon_equals	":="
+equals_greater	"=>"
 
 /*
  * "self" is the set of chars that should be returned as single-character
@@ -808,6 +809,11 @@ other			.
 					return COLON_EQUALS;
 				}
 
+{equals_greater}	{
+					SET_YYLLOC();
+					return EQUALS_GREATER;
+				}
+
 {self}			{
 					SET_YYLLOC();
 					return yytext[0];