Stash/controller model for staking (#1782)

* First steps to stash/controller separation

* More drafting

* More drafting

* Finish draft.

* Optimisation

* Remove accidental commit

* Make it build.

* Fix linked map for traits.

* Fix Option<_> variant.

*  Improve naming a tad

* Rebuild runtime

* Builds!

* First test.

* Bump RT version

* Minor fix

* Update Mock

* adds the correct reward testcase (+staking eras which was already ok)

* fixes the basic staking testcase to work properly (along with a small fix in the module)

* New logic to avoid controller transferring stash.

* Fix some build issues.

* adding some comments to tests

* Fix impls.

* adds a few more lines to explain the test case

* More fixes.

* gets the basic test up and running again

* Fix rest of build

* Rebuild wasm

* Fix docs.

* fix staking test with new chnages

* updating some tests, pending questions

* More working tests

* adds double staking test

* Docs

* remove invalid slashing test

* Payee stuff.

* Fix build

* Docs

* Fix test

* Fix a couple of tests

* Layout plan for finishing tests before Pragmen

* Add some working tests

* re-build staking and reward tests

* Add more tests

* fix offline grace test

* Nominator should have payee checked for cleanup

* adds more nomination tets

* adds validator prefs tests

* Fix and clean up some TODOs

* Fix a couple of issues

* Fix tests

* noting warnings from tests

* final fix of local tests

* Fix slot_stake bug

* Half baked test

* Add logic to limit `unstake_threshold` set in storage

* Make sure to check before writing!

Almost forgot this one

* Move a couple of comments

* fix last broken slot_stake test

* Ignore broken test