relax impure effect checking

The current effect checking pass adds the impure effect to any write to a non-local slot, including interior fields of a local slot. We can do better. At very least accept writes to local slots that don't traverse heap pointers; possibly try to do escape analysis and permit writes to non-escaping heap pointers.

1. WONTFIX (not required for bootstrapping).

   Graydon Hoare at 2011-01-27 01:55:46