Fix `integer-simple` API and tests

Fixes #16043.

• Support GHCi :print-ing of Integer from integer-simple. This requires adding an integer-simple Cabal flag to ghc.

• Fixes API discrepancies between integer-gmp and integer-simple

  • Added popCountInteger, bitInteger, gcdInteger, and lcmInteger
  • Made testBitInteger much more efficient

• Remove some now unnecessary CPP in base (ex: GHC.Natural no longer needs CPP for its export list)

• Fix GHCi printing tests that were now failing only due to integer-gmp having an extra CorePrep optimization (for turning small Integer's straight into applications of S#) by switching Integer to Int in a bunch of places