Move Externalities into its own crate (#3775)

* 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`

substrate/core/sr-primitives/Cargo.toml

@@ -16,6 +16,7 @@ runtime_io = { package = "sr-io", path = "../sr-io", default-features = false }
 log = { version = "0.4.8", optional = true }
 paste = "0.1.6"
 rand = { version = "0.7.2", optional = true }
+externalities = { package = "substrate-externalities", path = "../externalities", optional = true }
 impl-trait-for-tuples = "0.1.2"
 
 [dev-dependencies]
@@ -36,4 +37,5 @@ std = [
 	"primitives/std",
 	"app-crypto/std",
 	"rand",
+	"externalities",
 ]