This function executes the given closure in a context where the test
externalities are set. This makes the srml tests easier to write, as the
test externalities need to be created anyway.
* Move `Externalities` into `substrate-externalities`
- `Externalities` now support generic extensions
- Split of `primtives-storage` for storage primitive types
* Move the externalities scoping into `substrate-externalities`
* Fix compilation
* Review feedback
* Adds macro for declaring extensions
* Fix benchmarks
* Introduce `ExtensionStore` trait
* Last review comments
* Implement it for `ExtensionStore`
* impl
* patch
* lock
* some refactor
* some avoided copy
* new api without ref for doublemap
* fix
* version bump
* fix
* point to incoming release
* use codec latest
* bumpd impl version
* fix unused
* fix
* Update srml/support/src/storage/mod.rs
Co-Authored-By: Bastian Köcher <bkchr@users.noreply.github.com>