Serde configurable to [de]serialize with unit abbreviations.

[nicodemus26]

It would be excellent to have serde serialize to strings with unit abbreviations so that the types are carried in serialization.

[Aehmlo]

This should be pretty much trivial once #94 is implemented (#90 should be sufficient for deserialization for now), at least for the provided SI units.

[iliekturtles]

I'm also thinking making units first class in #107, which would make this easier. Currently working with base units isn't very friendly and de-serialization, as currently implemented, requires the resulting type to be provided rather than inferred during de-serialization.