[offchain] Support for sign & verify for crypto keys (#3023)

* Implement sign & verify.

* Use phrases and password.

* Sign & verify with authority keys.

* Fix tests.

* WiP

* WiP

* Allow the caller to decide on 'CryptoKind'.

* Remove TODO.

* Make seed private back.

* Fix non-std build and bump version.

* Use Into<u32> instead of asses.

* Add missing typedef.

substrate/core/primitives/src/sr25519.rs

@@ -379,6 +379,7 @@ fn derive_hard_junction(secret: &SecretKey, cc: &[u8; CHAIN_CODE_LENGTH]) -> Sec
 	secret.hard_derive_mini_secret_key(Some(ChainCode(cc.clone())), b"").0.expand()
 }
 
+/// The raw secret seed, which can be used to recreate the `Pair`.
 #[cfg(feature = "std")]
 type Seed = [u8; MINI_SECRET_KEY_LENGTH];