Support building with GHC 9.10 (base-4.20)

[cho-m]

Lifting the upper bound of base does allow build to succeed:

agda2hs/agda2hs.cabal

Line 63 in 7753783

build-depends: base >= 4.13 && < 4.20,

---

Opened this as working on updating to GHC 9.10.1 in Homebrew (Homebrew/homebrew-core#178590) and noticed the Homebrew agda formula failed to build with GHC 9.10.1 when building agda2hs.

[jespercockx]

It seems we should get rid of head from our codebase:

src/Agda2Hs/Compile.hs:132:139: error: [GHC-63394] [-Wx-partial, -Werror=x-partial]
    In the use of ‘head’
    (imported from Prelude, but defined in GHC.Internal.List):
    "This is a partial function, it throws an error on empty lists. Use pattern matching, 'Data.List.uncons' or 'Data.Maybe.listToMaybe' instead. Consider refactoring to use "Data.List.NonEmpty"."
    |
132 |         genericDocError =<< vcat (map (\x -> text $ "Cannot generate multiple constructors with the same identifier: " <> Hs.prettyPrint (head x)) duplicateCons)
    |                                                                                                                                           ^^^^