patch applied (ghc): Preserve type variable names during type inference

[Simon Peyton Jones]

Fri May  5 08:37:53 PDT 2006  simonpj at microsoft.com
  * Preserve type variable names during type inference
  
  During unification we attempt to preserve the print-names of type variables,
  so that type error messages tend to mention type variables using the 
  programmer's vocabulary.  
  
  This had bit-rotted a bit when I added impredicative polymorphism; especially
  when unBoxing a boxy type variable we should not gratuitously lose its name.
  

    M ./compiler/typecheck/TcUnify.lhs -13 +27