refactor!: make generateRandomMnemonic async (#222)

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Our HD keyring [has been
updated](MetaMask/accounts#100) to support
different kinds of cryptographic functions to randomly generate a
mnemonic, which may be an asynchronous operation in some cases.

This PR updates the `generateRandomMnemonic` on the `Keyring` type
accordingly, even though `eth-hd-keyring` does not implement the type
yet, because `KeyringController` uses it as keyring interface.

src/keyring.ts

@@ -256,8 +256,10 @@ export type Keyring<State extends Json> = {
   /**
    * Generates the properties for the keyring based on the given
    * BIP39-compliant mnemonic.
+   *
+   * @returns A promise resolving when the keyring has generated the properties.
    */
-  generateRandomMnemonic?(): void;
+  generateRandomMnemonic?(): Promise<void>;
 
   /**
    * Destroy the keyring.