* move inferred error sets into InternPool.
   - they are now represented by pointing directly at the corresponding
     function body value.
 * inferred error set working memory is now in Sema and expires after
   the Sema for the function corresponding to the inferred error set is
   finished having its body analyzed.
 * error sets use a InternPool.Index.Slice rather than an actual slice
   to avoid lifetime issues.