unit: clean up unit package

[btracey]

The unit package has a lot of problems. These should be cleaned up. Here are my suggestions

• Clean up basic documentation so that it actually matches the code
• Remove the (incorrect) comment examples and make them test Examples
• Do not provide so many constants for the units. Not unit.Millimeter, instead just supply a unit.Milli constant, so it can be unit.Milli * unit.Meter. Slightly less pretty, but drastically reduces the API surface and makes it more composable with outside code
• Redefine the Unit combination methods to match the mat types. For example, Add should take in two arguments and store to the receiver
• Remove the formatted field from Unit. It introduces unnecessary state
• Update the autogeneration to use go generate.
• It seems like the formatting code for unit types needs fixing.

[kortschak]

I'm not sure about the 4 point here. There is only a concrete Unit type, so it's reasonable to be able to chain things.

[btracey]

Will ponder.

[kortschak]

What is the plan for the formatted field?

[kortschak]

I've been thinking more about the formatted field and I think it should stay. It saves potentially large numbers of allocations (one per print). The race that can exist can be fixed by addition of a sync.RWMutex protecting it.

Related to the combination behaviour point. The zero value should be usable for these; if the dimensions field is nil (as opposed to empty), they should be set to the appropriate state.

[btracey]

I think all that's left of this is point 3

[kortschak]

Agreed