Rule.order should be set as the index of each expansion with rules
of the same name (e.g: a : b    # rule.order 1 | c    # rule.order 2).

As discussed in issue #378, when an embedded transformer (that is, one
passed to the Lark class using the transformer argument), is an
inplace transformer (either a subclass of Transformer_InPlace, or with
the @v_args(tree=True) decorator), the in-place transformer was not
working correctly and in-fact Lark used it like a normal non-in-place
transformer, expecting it to return the transformed value.