Proposal for removing separate `Nat` in favour of promoted `Natural` type.

[Rinat Stryungis]

Hello, libraries!

I want to present a proposal for changes in the `Base` library.
The changes in my proposal partially solve the following issue:
    https://gitlab.haskell.org/ghc/ghc/-/issues/10776
and remove the separate built-in kind `Nat` in favor of promoted type
`Natural`.

It means that previously one can't directly promote a data type with
Natural fields:

    data MyPointN = PointN Natural Natural -- could not be promoted
    data MyPointP = PointP Nat Nat         -- could be promoted, but
uninhabited in terms

    type M = PointP 1 10

but now one could promote the `Natural` data type:

    data MyPoint = Point Natural Natural
    type MyTLPoint1 = Point 1 10

The proposed changes both simplify the internals of the GHC by removing
separate kind and related things and make using of the type-level
naturals more convenient for users.

Also new type synonym

    type Nat = Natural

appeared in the Data.Type.TypeNats in the name of backward compatibility.

I've opened a new MR with the patch. In the patch with the already
implemented promotion of Naturals, one could find new and updated tests and
docs.

Also, I want to say about the breakages:

1. One must enable `TypeSynonymInstances` in order to define instances for
`Nat`.
2. Different instances for `Nat` and `Natural` won't type check anymore.
3. Type checker plugins that work with the natural numbers now
   should use `naturalTy` kind instead of removed `typeNatKind`

Anyone interested is welcome to look at the MR and discuss the proposal and
its implementation.

https://gitlab.haskell.org/ghc/ghc/-/merge_requests/3583

The latter is very short and easy, thanks to the recently merged patch with
a new `ghc-bignum` library. It greatly simplified my work.

Thanks and best regards.
Rinat Stryungis.
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