chore: Add missing [Register] for inductive types in Datatypes.v
chore: Add missing [Register] for inductive types in Datatypes.v

We try to consistently register inductive types defined in the
[Coq.Init.Datatypes] module, so that they can be fetch using
[Coqlib.ref_lib]. We follow a naming scheme consistent with the rest
of the module, that is: `core.<type name>.type' for the type, and
`core.<type name>.<constructor name>' for the constructors.

ejgallego/coq,gares/coq,coq/coq,JasonGross/coq,gares/coq,ejgallego/coq,coq/coq,SkySkimmer/coq,gares/coq,coq/coq,JasonGross/coq,Zimmi48/coq,Zimmi48/coq,SkySkimmer/coq,gares/coq,ejgallego/coq,SkySkimmer/coq,silene/coq,JasonGross/coq,gares/coq,coq/coq,silene/coq,ejgallego/coq,Zimmi48/coq,JasonGross/coq,Zimmi48/coq,SkySkimmer/coq,silene/coq