Fix argument injection in feature_columns()

[lionel-]

• Remove use of deprecated quo_expr(). It was renamed in rlang 0.2.0 to quo_squash() to make it clear that it's not a quosure accessor. Instead, it recursively unwraps all quosures in the expression tree. It seems the quosure accessor quo_get_expr() is the intended function here.

• Replace unsafe use of node_car() and node_cdr() with a small util call_inject_args(). Taking the CAR or CDR on something else than calls will crash R, and the arguments were not type checked.