patch applied (ghc): Tidy up substitutions

[Simon Peyton Jones]

Mon Nov  6 07:59:01 PST 2006  simonpj at microsoft.com
  * Tidy up substitutions
  
  The new simplifer stuff exposed the fact that the invariants on the
  TvSubstEnv and IdSubstEnv were insufficiently explicit.  (Resulted in
  a bug found by Sam Brosnon.)
  
  This patch fixes the bug, and tries to document the invariants pretty
  thoroughly. See 
  	Note [Extending the TvSubst] in Type
  	Note [Extenting the Subst]   in CoreSubst
  
  (Most of the new lines are comments.)
  

    M ./compiler/coreSyn/CoreSubst.lhs -24 +52
    M ./compiler/simplCore/SimplEnv.lhs +5
    M ./compiler/types/Type.lhs -8 +44