Haskell printer generator should not use deprecated OverlappingInstances extension

[andreasabel]

Haskell printer generator should not use the deprecated OverlappingInstances extension on recent GHCs. E.g., on ghc-8.2:

CPP/Print.hs:1:33: warning:
   -XOverlappingInstances is deprecated: instead use per-instance pragmas OVERLAPPING/OVERLAPPABLE/OVERLAPS
 |
1 | {-# LANGUAGE FlexibleInstances, OverlappingInstances #-}
 |                                 ^^^^^^^^^^^^^^^^^^^^

For backwards compatiblity, we could use #if pragmas.

From ghc-8.6 on, OverlappingInstances may trigger an error.

[andreasabel]

I am changing the Haskell backend to use the {-# OVERLAPPABLE #-} pragma.
As a consequence, BNFC-2.8.3 will only work for GHC >= 7.10.