Support SingularNameFor in (quantity) unit slots

[chiphogg]

This doesn't work, but it should (https://godbolt.org/z/bY3r1bKzc):

constexpr auto ONE_REV = make_constant(revolution);
QuantityD<Radians> two_pi_rad = ONE_REV;

[hoffbrinkle]

Oh, I stumbled across this a while ago and didn't realize it was a bug. Excited to see it fixed though!

[chiphogg]

I think the omission was originally intentional, because I wanted people to write .in(meters), not .in(meter). I still do. However, it's just a stylistic consideration. And as the example here shows, sometimes that stylistic consideration cuts in the other direction --- I found make_constant(revolution) easier to read than make_constant(revolutions), in terms of English grammar.

So now I think it's fine to enable this mapping, and just trust people's intrinsic incentives to write code that reads well. (This is in the same spirit as the earlier change to let people multiply and divide quantity makers. meters / seconds works now, although when Au was open sourced, meters / second was the only option.)

[chiphogg]

Did not mean to reopen. 😛