missing `package object double extends DoubleInstances`

[erikerlandson]

https://github.com/typelevel/cats/blob/41b99ed87ab56b32c7b793530aaeeaf5957a236e/algebra-core/src/main/scala/algebra/instances/double.scala#L8

[johnynek]

I don't know that we ever had that, but I don't see a major reason not to add it if that pattern exists in algebra.

The fact that these are not lawful under standard equality may also give some folks pause.

[erikerlandson]

Maybe it's deliberate. I just thought it was a little unintuitive that you can import from all or int, but double and float cannot be imported individually.