FIX - WCPS let clause should come before where clause

[Bang Pham Huu]

In the doc, it says:

FOR-CLAUSE
LET $variable := assignment [ , $variable := assignment ]
...
[ WHERE-CLAUSE ]
RETURN-CLAUSE

but in the WCPS parser code, it was

wcpsQuery : (forClauseList) 
            (whereClause)? 
            (letClauseList)? 
            (returnClause)