[CArray] Refactor bind_pos_boxed/at_pos_boxed.
[CArray] Refactor bind_pos_boxed/at_pos_boxed.

Since we've added child_objs, we can cache the generated String objects as
well, and eliminate the need for at/bind_pos_complex.

cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp