GHC 8.10 support - #210

Hi @fisx, @phadej,

This PR intended to close #208.

Details

• Relax boundaries to allow build with GHC 8.10.1.
• Bump LTS to enable build via stack with GHC 8.8.1 by default.
• Resolve test fail:

  Test suite doctests: RUNNING...
  
  ./swagger2/src/Data/Swagger/Declare.hs:165:46: warning: [-Wdeprecations]
      In the use of type constructor or class ‘ListT’
      (imported from Control.Monad.List, but defined in Control.Monad.Trans.List):
      Deprecated: "This transformer is invalid on most monads"
      |
  165 | instance MonadDeclare d m => MonadDeclare d (ListT m) where
      |                                              ^^^^^
  Test suite doctests: FAIL
  

I have enabled stylish-haskell by default. If you disagree with auto-formatting, I will undo formatting changes.

Thanks,
@swamp-agr