Add FromDhall instances for Int{,8,16,32,64}

[jchia]

#2348

[Gabriella439]

It looks like you will also need to fix this now that Dhall.Marshall.Internal re-exports Int{8,16,32,64}:

src/Dhall/Marshal/Decode.hs:149:1: error: [-Wunused-imports, -Werror=unused-imports]
    The import of ‘Data.Int’ is redundant
      except perhaps to import instances from ‘Data.Int’
    To import instances alone, use: import Data.Int()
    |
149 | import Data.Int                         (Int16, Int32, Int64, Int8)
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[jchia]

Fixed.

[Gabriella439]

Thank you! 🙂