* read head-data directly out of WASM memory
* implement ext_post_message for parachain WASM
* further refactoring of the parachain module
* add externalities error type
* accumulate posted messages when validating parachain candidate
* define Extrinsic type in primitives
* availability-store: store extrinsic data
* compute extrinsic and check against candidate
* add some egress queue tests
* grumbles & substrate update
* ensure everything builds