This implements Proposal 0475, introducing the `ListTuplePuns` extension
which is enabled by default.

Disabling this extension makes it invalid to refer to list, tuple and
sum type constructors by using built-in syntax like `[Int]`,
`(Int, Int)`, `(# Int#, Int# #)` or `(# Int | Int #)`.
Instead, this syntax exclusively denotes data constructors for use with
`DataKinds`.
The conventional way of referring to these data constructors by
prefixing them with a single quote (`'(Int, Int)`) is now a parser
error.

Tuple declarations have been moved to `GHC.Tuple.Prim` and the `Solo`
data constructor has been renamed to `MkSolo` (in a previous commit).
Unboxed tuples and sums now have real source declarations in `GHC.Types`.
Unit and solo types for tuples are now called `Unit`, `Unit#`, `Solo`
and `Solo#`.
Constraint tuples now have the unambiguous type constructors `CTuple<n>`
as well as `CUnit` and `CSolo`, defined in `GHC.Classes` like before.

A new parser construct has been added for the unboxed sum data
constructor declarations.

The type families `Tuple`, `Sum#` etc. that were intended to provide
nicer syntax have been omitted from this change set due to inference
problems, to be implemented at a later time.
See the MR discussion for more info.

Updates the submodule utils/haddock.
Updates the cabal submodule due to new language extension.

    Metric Increase:
        haddock.base

    Metric Decrease:
        MultiLayerModulesTH_OneShot
        size_hello_artifact

Proposal document: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0475-tuple-syntax.rst

Merge request: ghc/ghc!8820

Tracking ticket: ghc/ghc#21294