transformCreateStmt should put Ident nodes, not ColumnDef nodes, into

transformCreateStmt should put Ident nodes, not ColumnDef nodes, into
keys lists of Constraint nodes.  This eliminates a type pun that would
probably have caused trouble someday, and eliminates circular references
in the parsetree that were causing trouble now.
Also, change parser's uses of strcasecmp() to strcmp().  Since scan.l
has downcased any unquoted identifier, it is never correct to check an
identifier with strcasecmp() in the parser.  For example,
	CREATE TABLE FOO (f1 int, UNIQUE("F1"));
was accepted, which is wrong, and xlateSqlFunc did more than it should:
select datetime();
ERROR:  Function 'timestamp()' does not exist
(good)
select "DateTime"();
ERROR:  Function 'timestamp()' does not exist
(bad)