This is basically a hack that prevents the inclusion of `:code` in the
storage proof. A proper fix requires some changes to the trie crate to
not include nodes that aren't read.
* Introduce `CollectCollationInfo` runtime api
Instead of using well known keys to communicate information about a
collation between the runtime and the collator, we now use a runtime api
for this.
* Fixes bug
* Apply suggestions from code review
Co-authored-by: Sergei Shulepov <sergei@parity.io>
* Doc update
Co-authored-by: Sergei Shulepov <sergei@parity.io>