Separate PprFlags (used by Outputable) from DynFlags

At the moment, SDoc computations have full access to the entirety of DynFlags, despite only a minusculely small amount of the data structure being relevant to them. This proposal is to split out a PprFlags structure which will be contained in a DynFlags, and contain dynamic flags JUST for pretty-printing. There will be a function pprFlags :: DynFlags -> PprFlags. This also helps eliminate the circular dependency between DynFlags and Outputable.

This structure is not complete, but it should give a sense for some of the things that need to be put in here:

data PprFlags = PprFlags {
  pprUserLength         :: Int,
  pprCols               :: Int,
       
  useUnicode      :: Bool,
  useUnicodeSyntax :: Bool,
  targetPlatform :: Platform,
       
  suppressUniques :: Bool,                                                         
  errorSpans :: Bool,                                                              
  suppressModulePrefixes :: Bool,                                                  
  printExplicitKinds :: Bool,                                                      
  printExplicitForalls :: Bool,                                                  
  suppressTypeSignatures :: Bool,                                                
  suppressIdInfo :: Bool,                                                        
  suppressCoercions :: Bool,                                                     
  pprCaseAsLet :: Bool,                                                          
  pprShowTicks :: Bool,                                                          
  suppressTypeApplications :: Bool,                                              
  sccProfilingOn :: Bool, 

    }

I have a patch in-progress which factors this out, but I want to get some sign-offs that this is a good refactoring before I finish it.

The most annoying bits are pretty-printing in the code generator, where all of the platform constants (e.g. word-size) are necessary to print the right way.