chore(algebra/group/to_additive): map_namespace should make a meta constant (#1409)
chore(algebra/group/to_additive): map_namespace should make a meta constant (#1409)

* chore(algebra/group/to_additive): map_namespace should make a meta constance

`map_namespace` now produces a `meta constant` instead of a constant. This means that after importing `group_theory/coset` and typing `#print axioms`, `quotient_group._to_additive` is not in the list, since it is now a `meta constant`. This is a little bit neater, and it doesn't look like we're adding any axioms.

* Update to_additive.lean

* Update to_additive.lean

leanprover-community/mathlib,leanprover-community/mathlib