Issue 39

There may still be some rough edges here. In particular, it feels like `CacheWriter.write(K, V)` needs to be split into insert and update, and return a value. Also, an attempt was made to whip compute/write functions for bulk CRUD methods into shape, to blaze the trail for fixing #110... not super convinced they are correct now though.
Right, I guess we'd need to maintain a `Set` of entries written out or something. The issue with the `memoize` as it is, is that it doesn't account for arg values...