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/storage/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "substrate-primitives-storage"
+version = "2.0.0"
+authors = ["Parity Technologies <admin@parity.io>"]
+edition = "2018"
+description = "Storage related primitives"
+
+[dependencies]
+rstd = { package = "sr-std", path = "../../sr-std", default-features = false }
+serde = { version = "1.0.101", optional = true, features = ["derive"] }
+impl-serde = { version = "0.2.1", optional = true }
+
+[features]
+default = [ "std" ]
+std = [ "rstd/std", "serde", "impl-serde" ]