Add UnboxedTuples to Generics/SOP/Instances.hs

[monoidal]

GHC commit https://github.com/ghc/ghc/commit/fbe3fe003ac8d4a06 changed the type CodeBuffers to an unboxed tuple. As a result, generics-sop needs an extension to compile on head:

src/Generics/SOP/Instances.hs:298:1: error: [GHC-19590]
    • Illegal unboxed tuple type as function argument:
      (# State# RealWorld, CodingProgress, Buffer from, Buffer to #)
    • In the expansion of type synonym ‘GHC.IO.Encoding.Types.CodeBuffer#’
      In the type instance declaration for ‘Generics.SOP.Universe.Code’
      In the instance declaration for
        ‘Generics.SOP.Universe.Generic (BufferCodec from_ap4y to_ap4z state_ap4A)’
    Suggested fix: Perhaps you intended to use UnboxedTuples
    |
298 | deriveGeneric ''BufferCodec -- new
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^

[monoidal]

Superseded by #173