Update default-language & avoid default-extensions (#3880)

This upgrades the `default-language: Haskell98` to `Haskell2010`
and removes `default-extensions: RankNTypes, FlexibleContexts`
in favor of adding `LANGUAGE` pragmas where needed.

Moroever, this also drops `LANGUAGE` pragmas which have become redundant
due to `Haskell2010` (specifically, `EmptyDataDecls`,
`ForeignFunctionInterface` and `PatternGuards`)

Finally, an `other-extensions` specification is put in place for the
`Cabal` library component.

This helps loading up files directly in GHCi, such as e.g. `ghci Setup.hs`
without having to specify `-X...` flags.