Re-introduce zero copy codec and add minimal polkadot client API which uses linked native runtime (#65)

* client-api type and move duty roster types to primitives

* tuple implementation for slicable

* mild cleanup of deserialization code

* stubs which handle encoding and decoding themselves

* fancier impl_stubs macro

* zero-copy slicable API

* minimal polkadot-client API

* fix WASM API generation

* move native environment stuff to substrate executor

* fix warnings and grumbles

* BlockId in client interface

* Sync fixes and tests

* Updated to latest primitives

* Updated dependencies

* Updated for new (old) primitives

* Network as workspace member

* substrate-network

* Removed obsolete file

* begin_transaction on hash

Fix warning and directory restructure

Split up substrate-executor into polkadot-executor.

- Everything polkadot becomes polkadot-.
- Wasm (substrate) executor tests split from Wasm (Polkadot) runtime and
built independently.

Start isolating Substrate code from Polkadot

No tests yet.
Wasm build still pulls in std.

Bottom half minus tests builds.