* Add `get_whitelist` api

* add whitelisted caller

* Whitelist caller

* remove caller 0

* initial piping of origin (not actual value yet)

* remove attempt to pass origin around

* Add whitelist for `DidUpdate` storage on `pallet_timestamp`

* fix traits

* only add to whitelist if !contains

* PassBy not implemented error

* Whitelist read/writes explicitly per key

* update docs

* reduce trait constraint

* copy pasta

* Apply suggestions from code review

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>
Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>

* rename functions @apopiak

* missed some renaming

* enable doc tests

* Update docs

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>
Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>