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          Fix an mdo bug
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Embarassingly, this bug makes GHC either panic (for some programs) or
go into a loop (on others) in a recursive mdo that involves a
polymorphic function.  Urk!

The fix is twofold:
  a) add a missing bindInstsOfLocalFuns to tcStmtAndThen (RecStmt case)
  b) bind the correct set of variables in dsRecStmt

I added some explanatory comments about RecStmt in HsExpr too.

The tests is mdo/should_compile/mdo006