Replaces two copies of the same code, in Field::unnest_attrs and
Options::unnest_attrs.

Also, replaces the double specification of the output attributes list
variables in each of these functions (once when calculating unnest,
and once when copying the fields to every output).

Discussion was here, et seq:
  colin-kiegel#241 (comment)

Co-authored-by: Ted Driggs <ted.driggs@outlook.com>

As suggested in
  https://github.com/colin-kiegel/rust-derive-builder/pull/243/files/1db242c5a6b0c4066c58b48408e2ccd0da8ee8f9#diff-fce05abe2a1a48a344afbebd38b64914fde1d4131faa01fa9d32e5804b6f31ee