Proposal: Add Data instance for Const

[Ryan Scott]

GHC Trac #12438 [1] exists because there's no Data instance for Const
in base. I found this quite surprising, since we have Data instances
for just about every other type combinator out there (Identity, Sum,
Product, Compose, etc.), but not for Const. The fix for #12438 would
be quite simple: I propose we add

    deriving instance (Data a, Data b) => Data (Const a b)

to Data.Data in base. Any objections?

Ryan S.
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[1] https://ghc.haskell.org/trac/ghc/ticket/12438