Thanks to ghc-bignum, the compiler can be simplified:

* Types and constructors of Integer and Natural can be wired-in. It
  means that we don't have to query them from interfaces. It also means
  that numeric literals don't have to carry their type with them.

* The same code is used whatever ghc-bignum backend is enabled. In
  particular, conversion of bignum literals into final Core expressions
  is now much more straightforward. Bignum closure inspection too.

* GHC itself doesn't depend on any integer-* package anymore

* The `integerLibrary` setting is gone.