Optional storage entries (#75)

* Block builder (substrate)

* Fix wasm build

* Bulid on any block

* Test for block builder.

* Block import tests for client.

* Tidy ups

* Repotted client

* Avoid pointless work

* All backend stuff now manages optional storage.

Also simplified a lot of the backend.

* Native runtime-io now supports empty storage items.

* Finish up the API transition.

* Build fix.

* Fix tests.

* Remerge in changes to client.

* Final fixes.

* Unrevert typos

* Remove accidentally committed change

* Bring back zero copy

* Fix merge.