allow condition to be a block: by calling visit_expr of the visitor directly on the condition instead of walk_expr on the whole expression, we bypass the match to ExprWhile that calls visit_expr on the condition and visit_block on the body. This allow to re-enable visit_block in the visitor, as it won't be called on the while body
allow condition to use static variables: maintain a list of static variables used, and if they are mutable
