ipa::phoneme -- reorganise the feature list

This change:

  1.  Groups the different features together in bit order.

      This makes it easier to identify the bit coverage of all the
      features.

  2.  Groups the ranged values together by range type.

      This makes it easier to see the values available in a given
      range group.

  3.  Aligns the values within a group.

      This makes it easier to compare the values across the group
      (especially the set of features).

  4.  Add comments for missing feature bits.

      This makes it clearer (a) which bits are missing and (b) how
      many there are.