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-io/Cargo.toml

@@ -15,9 +15,9 @@ codec = { package = "parity-scale-codec", version = "1.0.0", default-features =
 hash-db = { version = "0.15.2", default-features = false }
 libsecp256k1 = { version = "0.3.0", optional = true }
 tiny-keccak = { version = "1.5.0", optional = true }
-environmental = { version = "1.0.2", optional = true }
 substrate-state-machine = { path = "../state-machine", optional = true }
 trie = { package = "substrate-trie", path = "../trie", optional = true }
+externalities = { package = "substrate-externalities", path = "../externalities", optional = true }
 
 [features]
 default = ["std"]
@@ -27,10 +27,10 @@ std = [
 	"rstd/std",
 	"hash-db/std",
 	"trie",
-	"environmental",
 	"substrate-state-machine",
 	"libsecp256k1",
 	"tiny-keccak",
+	"externalities",
 ]
 nightly = []
 strict = []