Unification of Nat and Naturals

This commit removes the separate kind 'Nat' and enables promotion
of type 'Natural' for using as type literal.
It partially solves #10776

Now the following code will be successfully typechecked:
    data C = MkC Natural
    type CC = MkC 1

Before this change we had to create the separate type for promotion
    data C = MkC Natural
    data CP = MkCP Nat
    type CC = MkCP 1

But CP is uninhabited in terms.

For backward compatibility type synonym `Nat` has been made:
    type Nat = Natural

The user's documentation and tests have been updated.
The haddock submodule also have been updated.

1 files changed, 0 insertions, 0 deletions

diff --git a/utils/haddock b/utils/haddock
-Subproject 6f16399e0320d0ef5e6c3dd0329ce7ed3715b6b
+Subproject f7d9e0bb987ca31c3b15cbe63198dafbeee3a39