Contract restoration (#2502)

* partial impl

* more checks

* improve TestExternalities + tests

* make tombstone raw to derive Eq

* remove before merge

* add test

* fmt

* update test

* doc

* bump version

* fix testing

* update runtime

* Fix TrieIdGenerator

* fix dummy trie id generator

* update test

* bump version

* format

* Update core/state-machine/src/testing.rs

Co-Authored-By: cheme <[email protected]>

* document test

* Apply suggestions from code review

Co-Authored-By: DemiMarie-temp <[email protected]>

* refactor

* fix

* fmt

* address review

* impl last_write

* Fix storage size, test, remove size in tombstone

* fix

* Update srml/contract/src/lib.rs

Co-Authored-By: Sergei Pepyakin <[email protected]>

* comment

* child_storage_root as &[u8]