Make runtime api calls native when possible (#1302)

* Add simple benchmark for the runtime api

* Make the executor support native calls

* Some documentation

* Hide behind `feature = "std"`

* Rework the native calls

* Make all tests compile again

* Make every parameter using the Block serialized/deserialized in the native call

* Forward `UnwindSafe` requirement

* Remove debug stuff

* Add some documentation

* Fixes warnings

* Fixes errors after master rebase

* Fixes compilation after master rebase

* Fixes compilation after rebase

substrate/core/sr-primitives/src/lib.rs

@@ -531,7 +531,7 @@ impl BasicInherentData {
 
 //TODO: https://github.com/paritytech/substrate/issues/1022
 /// Error type used while checking inherents.
-#[derive(Encode)]
+#[derive(Encode, PartialEq)]
 #[cfg_attr(feature = "std", derive(Decode))]
 pub enum CheckInherentError {
 	/// The inherents are generally valid but a delay until the given timestamp

substrate/core/sr-primitives/src/traits.rs

@@ -470,7 +470,7 @@ pub trait Header: Clone + Send + Sync + Codec + Eq + MaybeSerializeDebugButNotDe
 	type Number: Member + MaybeSerializeDebug + ::rstd::hash::Hash + Copy + MaybeDisplay + SimpleArithmetic + Codec;
 	type Hash: Member + MaybeSerializeDebug + ::rstd::hash::Hash + Copy + MaybeDisplay + Default + SimpleBitOps + Codec + AsRef<[u8]> + AsMut<[u8]>;
 	type Hashing: Hash<Output = Self::Hash>;
-	type Digest: Digest<Hash = Self::Hash>;
+	type Digest: Digest<Hash = Self::Hash> + Codec;
 
 	fn new(
 		number: Self::Number,