Bring back SubmitSignedTransaction trait. (#3908)

* Bring back SubmitSignedTransaction.

* Fix long lines.

* Add missing docs.

* Update core/primitives/src/crypto.rs

Co-Authored-By: Bastian Köcher <bkchr@users.noreply.github.com>

* Update node/runtime/src/lib.rs

Co-Authored-By: Bastian Köcher <bkchr@users.noreply.github.com>

* Update core/primitives/src/crypto.rs

Co-Authored-By: Bastian Köcher <bkchr@users.noreply.github.com>

substrate/core/primitives/src/crypto.rs

@@ -790,6 +790,9 @@ pub trait Pair: CryptoType + Sized + Clone + Send + Sync + 'static {
 		root.derive(path, Some(seed)).map_err(|_| SecretStringError::InvalidPath)
 	}
 
+	/// Interprets the string `s` in order to generate a key pair.
+	///
+	/// See [`from_string_with_seed`](Self::from_string_with_seed) for more extensive documentation.
 	fn from_string(s: &str, password_override: Option<&str>) -> Result<Self, SecretStringError> {
 		Self::from_string_with_seed(s, password_override).map(|x| x.0)
 	}

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

@@ -255,18 +255,39 @@ impl From<ed25519::Public> for MultiSigner {
 	}
 }
 
+impl TryFrom<MultiSigner> for ed25519::Public {
+	type Error = ();
+	fn try_from(m: MultiSigner) -> Result<Self, Self::Error> {
+		if let MultiSigner::Ed25519(x) = m { Ok(x) } else { Err(()) }
+	}
+}
+
 impl From<sr25519::Public> for MultiSigner {
 	fn from(x: sr25519::Public) -> Self {
 		MultiSigner::Sr25519(x)
 	}
 }
 
+impl TryFrom<MultiSigner> for sr25519::Public {
+	type Error = ();
+	fn try_from(m: MultiSigner) -> Result<Self, Self::Error> {
+		if let MultiSigner::Sr25519(x) = m { Ok(x) } else { Err(()) }
+	}
+}
+
 impl From<ecdsa::Public> for MultiSigner {
 	fn from(x: ecdsa::Public) -> Self {
 		MultiSigner::Ecdsa(x)
 	}
 }
 
+impl TryFrom<MultiSigner> for ecdsa::Public {
+	type Error = ();
+	fn try_from(m: MultiSigner) -> Result<Self, Self::Error> {
+		if let MultiSigner::Ecdsa(x) = m { Ok(x) } else { Err(()) }
+	}
+}
+
 #[cfg(feature = "std")]
 impl std::fmt::Display for MultiSigner {
 	fn fmt(&self, fmt: &mut std::fmt::Formatter) -> std::fmt::Result {