Refactor the runtime API to use traits. (#878)

* Add missing `As` imports.

* Adds new API traits that will be used by the client and runtime

* Switch consensus to new API's

* Switches transaction-pool to new API's

* Move runtime api stuff into its own crate

* Adds `impl_apis!` macro for implementing the new API traits

* Make `metadata` return directly a blob

* Runtime replace `impl_stubs!` with `impl_apis!`

* Switches to none feature based approach for declaring the different API traits

* Fixes compilation error

* Fixes errors

* Make the `decl_apis!` trait usable from the outside

* Make the `test-client` use the new API traits

* Remove last `impl_stubs!` bits and move some of them into wasm executor for tests

* A little bit more documentation