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Went through the TODOs, removed a bunch, which are outdated or nothing more than a regular comment, documented a bunch more as actual tickets and made them FIXMEs and unified their structure (`FIXME #TICKETNO DESC` for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

@@ -16,7 +16,7 @@
 
 //! Utility functions to interact with Substrate's Base-16 Modified Merkle Patricia tree ("trie").
 
-// TODO: no_std
+// FIXME: no_std - https://github.com/paritytech/substrate/issues/1574
 
 mod error;
 mod node_header;
@@ -71,7 +71,7 @@ pub fn delta_trie_root<H: Hasher, I, A, B>(db: &mut HashDB<H>, mut root: H::Out,
 		for (key, change) in delta {
 			match change {
 				Some(val) => trie.insert(key.as_ref(), val.as_ref())?,
-				None => trie.remove(key.as_ref())?, // TODO: archive mode
+				None => trie.remove(key.as_ref())?,
 			};
 		}
 	}
@@ -151,7 +151,7 @@ pub fn child_delta_trie_root<H: Hasher, I, A, B>(_storage_key: &[u8], db: &mut H
 		for (key, change) in delta {
 			match change {
 				Some(val) => trie.insert(key.as_ref(), val.as_ref())?,
-				None => trie.remove(key.as_ref())?, // TODO: archive mode
+				None => trie.remove(key.as_ref())?,
 			};
 		}
 	}