Provide more options for rendering units

[propensive]

For any particular unit, certain conventions exist for how that unit should be rendered, for example,

• the order the component units should appear in
• whether it should be written with metric multipliers (e.g. k, M, G, T, etc)
• which multipliers should be used, e.g. ns, µs, ms and s are all commonly used for times, but rarely ks or cs, whereas lengths are often stated in mm, cm, m, km, but rarely Mm or Gm.

This can be provided by an erased typeclass instance, though its state/format still needs to be determined.