Went through the TODOs, removed a bunch, which are outdated or nothing more than a regular comment, documented a bunch more as actual tickets and made them FIXMEs and unified their structure (`FIXME #TICKETNO DESC` for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.
* Implement a non generic version of child delta trie
* Use delta_trie_root in state_machine
* Expand InMemory backend to support multi-storage
* Create Consolidate trait
* Fix all crate compile and remove unused OverlayedChanges::drain
* Implement child storage root support and overlay changes
* Add child storage reader
* Add child storage writer
* Implement child storage cleaning
* Fix light backend compile
* Add all required ext functions for wasm executor
* Add ext def to io
* Add all io functions
* Fix nostd compile
* Add simple test
* Remove unnecessary vec copy in child_storage_root_transaction
* Use values_mut/for_each to make it shorter
* Use extend to shorter a for loop
* Move record_all_keys to trie so it's easier to generic them later
* space -> tab
* Remove to_owned in debug format
* Clean out all to_owned
* Break debug_trace to multiple lines
* Remove 0..
* UserError copy/paste typo
* Replace Vec::from_raw_parts by slice::from_raw_parts
* Use iter::empty()
* Wrap some long lines
* Wrap a missing line
* Remove unnecessary map
https://github.com/paritytech/substrate/pull/856#discussion_r226222663
* Call ext_free after from_raw_parts
* Fix tests in other crates
Stanislav Tkach