For the universal parser, `rb_reg_named_capture_assign_iter_impl`
function is shared between the parser and ripper.  However
`parser_params` struct is partially different, and `assignable`
function depends on that part indirectly.

rb_ary_new function was not used by the parser and could be removed.

Changed to use `rb_parser_is_ascii_string` function instead of `is_ascii_string` function