Generate an enum with one variant for each unit, use that for iteration.

[bheisler]

This extends the quantity! macro to generate a non-exhaustive enum with one variant per unit. A static slice of the different potential values of this enum is also generated, which is exposed publicly through a units() function, which returns an iterator. This can be used to iterate over the different units.

[adamreichold]

This indeed much simpler than doing tricks with HList! I think a test case that exercises the API would be nice.

[iliekturtles]

Thanks for the PR!

[bheisler]

I think I've fixed everything.

[iliekturtles]

Couple more minor changes. I think the code looks good so far but I have only reviewed through github. I want to do one more review, hopefully this weekend, with the compiler then I think we'll be able to merge.

[iliekturtles]

I haven't forgotten about this, I just didn't finish my review this weekend. I like using $plural for the doc comment and think it would be good to switch struct $unit to use the same value if there is no user-provided comment and use the user-provided comment on the enum.

[iliekturtles]

Merged! Thanks so much for the PR and sorry for the delays. My quest to use the user-provided documentation on the enum members ended up being a dead-end because you can't use a macro in place of an enum member. I will submit a separate PR to convert struct $unit to use $plural.