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/core/sr-api/Cargo.toml

@@ -0,0 +1,19 @@
+[package]
+name = "sr-api"
+version = "0.1.0"
+authors = ["Parity Technologies <admin@parity.io>"]
+
+[dependencies]
+parity-codec = { version = "2.0", default-features = false }
+sr-std = { path = "../sr-std", default-features = false }
+sr-primitives = { path = "../sr-primitives", default-features = false }
+sr-version = { path = "../sr-version", default-features = false }
+
+[features]
+default = ["std"]
+std = [
+	"sr-std/std",
+	"parity-codec/std",
+	"sr-primitives/std",
+	"sr-version/std",
+]