improve parser

>>> for i=0,i<5,i=i+1 then x=5 end
( walley_statements
    ( statements
        ( for)
        ( assignment( =(id i)(num 0)))
        ( simple_relation( <(id i)(num 5)))
        ( assignment( =(id i)( +(id i)(num 1))))
        ( statements( =(id x)(num 15)))
    )
 )

>>> while x<5 then x=x+1 end
 ( walley_statements
    ( statements
        ( while)
        ( simple_relation( <(id x)(num 5)))
        ( statements( =(id y)(num 12)))
    )
 )