Remove One (again)

[oxinabox]

Yet again be bring this up.
making a new issue as it was mixed in with everything else.
past discussions #62
#61 (comment)

It is barely a valid differential.
It's actually just a scaling value for putting infront of a differential.
Or for using to seed.

I think it might only be used in a few scalar rules, and there it can be replaced with true.
Since it is just there to makes sure we generate dx->dx() so we currently end up generatign dx->One()dx but we can happily also just generate dx->true*x.

The One() type doesn't give enough wins, and is confusing.

[willtebbutt]

We should remove it in the next breaking release.

edit: truth be told, I'd completely forgotten that we still have this.

[oxinabox]

closed in #360