Refactor containers h includes

[RikuMinamiyama]

Overview
Removed unnecessary CPP and C header includes.

Correction details

• C header removal

  The following modules use values defined in containers.h:

  /haskell-containers/containers/src/Data/Graph.hs
  → DEFINE_PATTERN_SYNONYMS, USE_ST_MONAD, USE_UNBOXED_ARRAYS
  
  /haskell-containers/containers/src/Data/Sequence.hs
  → DEFINE_PATTERN_SYNONYMS
  
  /haskell-containers/containers/src/Data/IntSet/Internal.hs
  → WORD_SIZE_IN_BITS
  
  /haskell-containers/containers/src/Data/Map/Internal.hs
  → WORD_SIZE_IN_BITS
  
  /haskell-containers/containers/src/Data/Sequence/Internal.hs
  → DEFINE_PATTERN_SYNONYMS
  
  /haskell-containers/containers-tests/tests/IntSetValidity.hs
  → WORD_SIZE_IN_BITS
  

  Removed #include "containers.h" from all other modules.

• CPP removal

  Removed CPP language pragmas from files that do not use CPP directives.

Testing

• cabal build containers succeeds
• cabal test containers-tests passes all tests
• cabal bench containers-tests passes all benchmarks

While the changes are straightforward, they affect a wide range of files, so careful review is requested.

Fixes #968