grandpa: missing equivocation reporting nits (#5953)

* primitives: move reporting key type to common key types

* session: remove useless methods on MembershipProof

* grandpa: remove std special-casing when checking signatures

* grandpa: add some more docs

* grandpa: use proper error types rather than strings

substrate/primitives/core/src/crypto.rs

@@ -1006,6 +1006,8 @@ pub mod key_types {
 	pub const AUTHORITY_DISCOVERY: KeyTypeId = KeyTypeId(*b"audi");
 	/// Key type for staking, built-in. Identified as `stak`.
 	pub const STAKING: KeyTypeId = KeyTypeId(*b"stak");
+	/// Key type for equivocation reporting, built-in. Identified as `fish`.
+	pub const REPORTING: KeyTypeId = KeyTypeId(*b"fish");
 	/// A key type ID useful for tests.
 	pub const DUMMY: KeyTypeId = KeyTypeId(*b"dumy");
 }