Document more TODOs as tickets (#1418)

Went through the TODOs, removed a bunch, which are outdated or nothing more than a regular comment, documented a bunch more as actual tickets and made them FIXMEs and unified their structure (`FIXME #TICKETNO DESC` for local tickets, `FIXME: DESC LINK` for external tickets) for easier in-editor support. Further more remove unnecessary remarks and related old code that I noticed in that instance.

substrate/core/primitives/src/changes_trie.rs

@@ -49,7 +49,7 @@ impl ChangesTrieConfiguration {
 			return 1;
 		}
 
-		// TODO: use saturating_pow when available
+		// FIXME: use saturating_pow once stabilized  - https://github.com/rust-lang/rust/issues/48320
 		let mut max_digest_interval = self.digest_interval;
 		for _ in 1..self.digest_levels {
 			max_digest_interval = match max_digest_interval.checked_mul(self.digest_interval) {

substrate/core/primitives/src/ed25519.rs

@@ -238,16 +238,6 @@ impl Pair {
 		r.copy_from_slice(pk);
 		Public(r)
 	}
-
-	/// Derive a child key. Probably unsafe and broken.
-	// TODO: proper HD derivation https://cardanolaunch.com/assets/Ed25519_BIP.pdf
-	pub fn derive_child_probably_bad(&self, chain_data: &[u8]) -> Pair {
-		let sig = self.sign(chain_data);
-		let mut seed = [0u8; 32];
-		seed.copy_from_slice(&sig[..32]);
-
-		Pair::from_seed(&seed)
-	}
 }
 
 /// Verify a signature on a message. Returns true if the signature is good.
@@ -350,12 +340,6 @@ mod test {
 		assert_eq!(pair1.public(), pair2.public());
 	}
 
-	#[test]
-	fn derive_child() {
-		let pair = Pair::generate();
-		let _pair2 = pair.derive_child_probably_bad(b"session_1234");
-	}
-
 	#[test]
 	fn ss58check_roundtrip_works() {
 		let pair = Pair::from_seed(b"12345678901234567890123456789012");