Improve Attribute and Base performance

This improve the performance of the Attribute and Base components by making algorithmic changes as well as micro-optimizations along the "hot" code paths.

### Todos

- [ ] Add description to this pull request.
- [ ] Add `ATTRS` mutation method to `Y.Base` which clears caches.
- [ ] Fix minor issue in complex attribute values, e.g. "foo.bar".
- [ ] Fix issue with attribute value cloning of nodes in IE.
- [ ] Add API and user guide docs for changes.
- [ ] Forecast minor back-compat breakage.