patch applied (ghc): Rewrite fixTvSubstEnv so it iteratively applies its substition

[Ian Lynagh]

Wed Feb 20 07:37:52 PST 2008  Ian Lynagh <igloo at earth.li>
  * Rewrite fixTvSubstEnv so it iteratively applies its substition
  This fixes a stack overflow when using strict UniqFMs. It might be
  possible to rewrite it more efficiently, or to avoid needing it in the
  first place.

    M ./compiler/typecheck/TcGadt.lhs -3 +7