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/primitives/Cargo.toml

@@ -31,6 +31,8 @@ num-traits = { version = "0.2.8", default-features = false }
 zeroize = "0.10.1"
 lazy_static = { version = "1.4.0", optional = true }
 parking_lot = { version = "0.9.0", optional = true }
+externalities = { package = "substrate-externalities", path = "../externalities", optional = true }
+primitives-storage = { package = "substrate-primitives-storage", path = "storage", default-features = false }
 
 [dev-dependencies]
 substrate-serializer = { path = "../serializer" }
@@ -52,7 +54,7 @@ std = [
 	"log",
 	"wasmi",
 	"lazy_static",
-    "parking_lot",
+	"parking_lot",
 	"primitive-types/std",
 	"primitive-types/serde",
 	"primitive-types/byteorder",
@@ -79,4 +81,6 @@ std = [
 	"schnorrkel",
 	"regex",
 	"num-traits/std",
+	"externalities",
+	"primitives-storage/std",
 ]