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

substrate/node/runtime/Cargo.toml

@@ -8,9 +8,10 @@ rustc-hex = "1.0"
 hex-literal = "0.1.0"
 serde = { version = "1.0", default-features = false }
 serde_derive = { version = "1.0", optional = true }
-safe-mix = { version = "1.0", default-features = false}
+safe-mix = { version = "1.0", default-features = false }
 parity-codec = "2.0"
 parity-codec-derive = "2.0"
+sr-api = { path = "../../core/sr-api", default-features = false }
 sr-std = { path = "../../core/sr-std" }
 sr-io = { path = "../../core/sr-io" }
 srml-support = { path = "../../srml/support" }
@@ -36,6 +37,7 @@ default = ["std"]
 std = [
 	"parity-codec/std",
 	"substrate-primitives/std",
+	"sr-api/std",
 	"sr-std/std",
 	"sr-io/std",
 	"srml-support/std",