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Went through the TODOs, removed a bunch, which are outdated or nothing more than a regular comment, documented a bunch more as actual tickets and made them FIXMEs and unified their structure (`FIXME #TICKETNO DESC` for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

@@ -24,8 +24,6 @@ use codec::{Codec, Decode, KeyedVec, Input};
 #[macro_use]
 pub mod generator;
 
-// TODO: consider using blake256 to avoid possible preimage attack.
-
 struct IncrementalInput<'a> {
 	key: &'a [u8],
 	pos: usize,