Add a Decorated class, which will be offline decorated Metadata (#1463)

* Move Metadata to api-metadata

* Make tests in api-metadata pass

* Make tests in api-metadata pass

* Refactor fromMetadata into decorated

* Fix some linting

* Replace headers

* Fix all tests

* Small lint

* Fix tests

* Fix metadata docs scripts

* Fix import ordering

* Add line breaks

* Grumbles

* Fix forgotten import

* rpcMetadata everywhere

* Fixlint

* s/api-metadata/metadata

* Fix latest tests

* Fix lint

* Fix tests

packages/metadata/src/Metadata/MagicNumber.ts

@@ -0,0 +1,17 @@
+// Copyright 2017-2019 @polkadot/metadata authors & contributors
+// This software may be modified and distributed under the terms
+// of the Apache-2.0 license. See the LICENSE file for details.
+
+import { assert } from '@polkadot/util';
+
+import U32 from '@polkadot/types/primitive/U32';
+
+export const MAGIC_NUMBER = new U32(0x6174656d); // `meta`, reversed for Little Endian encoding
+
+export default class MagicNumber extends U32 {
+  constructor (value?: any) {
+    super(value);
+
+    assert(this.eq(MAGIC_NUMBER), `MagicNumber mismatch: expected ${MAGIC_NUMBER.toHex()}, found ${this.toHex()}`);
+  }
+}