Implement change trie for child trie. (#3122)

* Initial implementation, some redundancy is awkward and there is
some useless computation (but there is a pending pr for that).
Next are tests.

* Minimal tests and fix extend child.

* implement iterator for change child trie.

* prune child trie.

* Fix pruning test.

* bump spec version.

* Avoid empty child trie (could also be checked before)

* tabs.

* Fix child digest overriding each others.

* Restore doc deleted on merge.

* Check correct child value on extrinsics build.

* Revert runtime version update.