Replace JSON metadata with custom metadata structures (#766)

* Move module metadata from json string to custom metadata

* Revisit the metadata structures

1. Move the structures into the metadata crate.
2. Switch to using Cow/MaybeOwnedArray to support Encode/Decode

* Adapt to new metadata structures

* Convert event json metadata to new metadata structures

* Convert storage json metadata to new metadata structures

* Convert runtime metadata from json to new metadata structs

* Implements new metadata structures in client and runtime

* Fixes errors on `no_std`

* Fixes errors after rebasing master

* Do not use `Cow` anymore in metadata

Also replace `String` with our own type definition `StringBuf`.
This fixes compilation on `no_std`.

* Wrap `RuntimeMetadata` in `RuntimeMetadataVersioned` to support versioning

* Move metadata into `srml` and make core unaware of the implementation

substrate/Cargo.toml

@@ -28,7 +28,6 @@ members = [
 	"core/executor",
 	"core/extrinsic-pool",
 	"core/keyring",
-	"core/metadata",
 	"core/misbehavior-check",
 	"core/network",
 	"core/primitives",
@@ -46,6 +45,7 @@ members = [
 	"srml/democracy",
 	"srml/example",
 	"srml/executive",
+	"srml/metadata",
 	"core/sr-primitives",
 	"srml/session",
 	"srml/staking",