Support printing `integer-simple` Integers in GHCi

This means that `:p` no longer leaks the implementation details of

`Integer` with `integer-simple`. The `print037` test case should

exercise all possible code paths for GHCi's code around printing

`Integer`s (both in `integer-simple` and `integer-gmp`).

`ghc` the package now also has a Cabal `integer-simple` flag (like the

`integer-gmp` one).