Finally, I managed to squash an infamous bug in :print

Finally, I managed to squash an infamous bug in :print

  It turns out the newtype handling code in :print
  was slipping non mutable Tyvars in the types reconstructed.
  The error message eventually produced was rather obscure:

  [src/Tp.hs:75:28-64] *MainTp> :p x
  *** Exception: No match in record selector Var.tcTyVarDetails
  [src/Tp.hs:75:28-64] *MainTp>

  Due to non mutable tyvars, unifyType was failing.
  A well placed assertion in the unifyType code would have made
   my life much easier.
  Which reminds me I should install a -ddump-* system in the
  RTTI subsystem, or future hackers will run away in swearing.

MERGE TO STABLE