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Went through the TODOs, removed a bunch, which are outdated or nothing more than a regular comment, documented a bunch more as actual tickets and made them FIXMEs and unified their structure (`FIXME #TICKETNO DESC` for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/state-machine/src/backend.rs

@@ -118,7 +118,7 @@ impl<H: Hasher> Consolidate for MemoryDB<H> {
 }
 
 /// Error impossible.
-// TODO: use `!` type when stabilized.
+// FIXME: use `!` type when stabilized. https://github.com/rust-lang/rust/issues/35121
 #[derive(Debug)]
 pub enum Void {}
 
@@ -291,7 +291,7 @@ impl<H: Hasher> Backend<H> for InMemory<H> where H::Out: HeapSizeOf {
 	}
 
 	fn try_into_trie_backend(self) -> Option<TrieBackend<Self::TrieBackendStorage, H>> {
-		let mut mdb = MemoryDB::default();	// TODO: should be more correct and use ::new()
+		let mut mdb = MemoryDB::default();
 		let mut root = None;
 		for (storage_key, map) in self.inner {
 			if storage_key != None {

substrate/core/state-machine/src/changes_trie/changes_iterator.rs

@@ -104,7 +104,7 @@ pub fn key_changes_proof_check<S: RootsStorage<H>, H: Hasher>(
 	max: u64,
 	key: &[u8]
 ) -> Result<Vec<(u64, u32)>, String> where H::Out: HeapSizeOf {
-	let mut proof_db = MemoryDB::<H>::default();	// TODO: use new for correctness
+	let mut proof_db = MemoryDB::<H>::default();
 	for item in proof {
 		proof_db.insert(&item);
 	}

substrate/core/state-machine/src/changes_trie/prune.rs

@@ -62,7 +62,7 @@ pub fn prune<S: Storage<H>, H: Hasher, F: FnMut(H::Out)>(
 	};
 
 	// delete changes trie for every block in range
-	// TODO: limit `max_digest_interval` so that this cycle won't involve huge ranges
+	// FIXME: limit `max_digest_interval` so that this cycle won't involve huge ranges
 	for block in first..last+1 {
 		let root = match storage.root(current_block, block) {
 			Ok(Some(root)) => root,

substrate/core/state-machine/src/ext.rs

@@ -302,7 +302,7 @@ where
 		);
 		let root_and_tx = root_and_tx.map(|(root, changes)| {
 			let mut calculated_root = Default::default();
-			let mut mdb = MemoryDB::default();	// TODO: use new for correctness
+			let mut mdb = MemoryDB::default();
 			{
 				let mut trie = TrieDBMut::<H>::new(&mut mdb, &mut calculated_root);
 				for (key, value) in changes {

substrate/core/state-machine/src/proving_backend.rs

@@ -195,7 +195,7 @@ where
 	H: Hasher,
 	H::Out: HeapSizeOf,
 {
-	let mut db = MemoryDB::default();	// TODO: use new for correctness
+	let mut db = MemoryDB::default();
 	for item in proof {
 		db.insert(&item);
 	}

substrate/core/state-machine/src/trie_backend.rs

@@ -82,7 +82,7 @@ impl<S: TrieBackendStorage<H>, H: Hasher> Backend<H> for TrieBackend<S, H> where
 	}
 
 	fn pairs(&self) -> Vec<(Vec<u8>, Vec<u8>)> {
-		let mut read_overlay = MemoryDB::default();	// TODO: use new for correctness
+		let mut read_overlay = MemoryDB::default();
 		let eph = Ephemeral::new(self.essence.backend_storage(), &mut read_overlay);
 
 		let collect_all = || -> Result<_, Box<TrieError<H::Out>>> {
@@ -106,7 +106,7 @@ impl<S: TrieBackendStorage<H>, H: Hasher> Backend<H> for TrieBackend<S, H> where
 	}
 
 	fn keys(&self, prefix: &Vec<u8>) -> Vec<Vec<u8>> {
-		let mut read_overlay = MemoryDB::default();	// TODO: use new for correctness
+		let mut read_overlay = MemoryDB::default();
 		let eph = Ephemeral::new(self.essence.backend_storage(), &mut read_overlay);
 
 		let collect_all = || -> Result<_, Box<TrieError<H::Out>>> {
@@ -193,7 +193,7 @@ pub mod tests {
 
 	fn test_db() -> (MemoryDB<Blake2Hasher>, H256) {
 		let mut root = H256::default();
-		let mut mdb = MemoryDB::<Blake2Hasher>::default();	// TODO: use new() to be more correct
+		let mut mdb = MemoryDB::<Blake2Hasher>::default();
 		{
 			let mut trie = TrieDBMut::new(&mut mdb, &mut root);
 			trie.insert(b"key", b"value").expect("insert failed");
@@ -230,7 +230,7 @@ pub mod tests {
 	#[test]
 	fn pairs_are_empty_on_empty_storage() {
 		assert!(TrieBackend::<MemoryDB<Blake2Hasher>, Blake2Hasher>::new(
-			MemoryDB::default(),	// TODO: use new() to be more correct
+			MemoryDB::default(),
 			Default::default(),
 		).pairs().is_empty());
 	}

substrate/core/state-machine/src/trie_backend_essence.rs

@@ -176,7 +176,7 @@ impl<'a,
 	where H::Out: HeapSizeOf
 {
 	fn keys(&self) -> HashMap<H::Out, i32> {
-		self.overlay.keys() // TODO: iterate backing
+		self.overlay.keys()
 	}
 
 	fn get(&self, key: &H::Out) -> Option<DBValue> {