Introduces author_hasKey and author_hasSessionKeys rpc endpoints (#4720)

* Introduces `author_hasKey` and `author_hasSessionKeys` rpc endpoints

Both endpoints can be used to check if a key is present in the keystore.

- `hasKey` works on with an individual public key and key type. It
checks if a private key for the given combination exists in the
keystore.
- `hasSessionKeys` works with the full encoded session key blob stored
on-chain in `nextKeys`. This requires that the given blob can be decoded
by the runtime. It will return `true`, iff all public keys of the
session key exist in the storage.

Fixes: https://github.com/paritytech/substrate/issues/4696

* Update client/rpc-api/src/author/error.rs

Co-Authored-By: Nikolay Volf <nikvolf@gmail.com>

* Indentation

Co-authored-by: Nikolay Volf <nikvolf@gmail.com>

substrate/primitives/core/src/crypto.rs

@@ -19,7 +19,6 @@
 // end::description[]
 
 use sp_std::hash::Hash;
-#[cfg(feature = "full_crypto")]
 use sp_std::vec::Vec;
 #[cfg(feature = "std")]
 use sp_std::convert::TryInto;
@@ -520,8 +519,7 @@ pub trait Public: AsRef<[u8]> + AsMut<[u8]> + Default + Derive + CryptoType + Pa
 	fn from_slice(data: &[u8]) -> Self;
 
 	/// Return a `Vec<u8>` filled with raw data.
-	#[cfg(feature = "std")]
-	fn to_raw_vec(&self) -> Vec<u8> { self.as_slice().to_owned() }
+	fn to_raw_vec(&self) -> Vec<u8> { self.as_slice().to_vec() }
 
 	/// Return a slice filled with raw data.
 	fn as_slice(&self) -> &[u8] { self.as_ref() }