Implement nested storage transactions (#6269)

* Add transactional storage functionality to OverlayChanges

A collection already has a natural None state. No need to
wrap it with an option.

* Add storage transactions runtime interface

* Add frame support for transactions

* Fix committed typo

* Rename 'changes' variable to 'overlay'

* Fix renaming change

* Fixed strange line break

* Rename clear to clear_where

* Add comment regarding delete value on mutation

* Add comment which changes are covered by a transaction

* Do force the arg to with_transaction return a Result

* Use rust doc comments on every documentable place

* Fix wording of insert_diry doc

* Improve doc on start_transaction

* Rename value to overlayed in close_transaction

* Inline negation

* Improve wording of close_transaction comments

* Get rid of an expect by using get_or_insert_with

* Remove trailing whitespace

* Rename should to expected in tests

* Rolling back a transaction must mark the overlay as dirty

* Protect client initiated storage tx from being droped by runtime

* Review nits

* Return Err when entering or exiting runtime fails

* Documentation fixup

* Remove close type

* Move enter/exit runtime to excute_aux in the state-machine

* Rename Discard -> Rollback

* Move child changeset creation to constructor

* Move child spawning into the closure

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Fixup for code suggestion

* Unify re-exports

* Rename overlay_changes to mod.rs and move into subdir

* Change proof wording

* Adapt a new test from master to storage-tx

* Suggestions from the latest round of review

* Fix warning message

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

substrate/primitives/state-machine/Cargo.toml

@@ -25,10 +25,13 @@ codec = { package = "parity-scale-codec", version = "1.3.1" }
 num-traits = "0.2.8"
 rand = "0.7.2"
 sp-externalities = { version = "0.8.0-rc3", path = "../externalities" }
+itertools = "0.9"
+smallvec = "1.4"
 
 [dev-dependencies]
 hex-literal = "0.2.1"
 sp-runtime = { version = "2.0.0-rc3", path = "../runtime" }
+pretty_assertions = "0.6.1"
 
 [features]
 default = []