message: refactor(core): use bip32util in baseCoin.signMessage
  Issue:
diff: import { BigNumber } from 'bignumber.js';
  import * as bitcoin from '@bitgo/utxo-lib';
  import { hdPath } from '../bitcoin';
  -const bitcoinMessage = require('bitcoinjs-message');
  +import * as bip32 from 'bip32';
  import * as Bluebird from 'bluebird';
  import { BitGo } from '../bitgo';
  import * as errors from '../errors';
  @@ -22,6 +22,7 @@ import { Enterprises } from './enterprises';
  // re-export account lib transaction types
  import { BaseCoin as AccountLibBasecoin } from '@bitgo/account-lib';
  +import { signMessage } from '../bip32util';
  export type TransactionType = AccountLibBasecoin.TransactionType;
  export interface TransactionRecipient {
  @@ -338,11 +339,7 @@ export abstract class BaseCoin {
  */
  signMessage(key: { prv: string }, message: string, callback?: NodeCallback<Buffer>): Bluebird<Buffer> {
  return co<Buffer>(function* cosignMessage() {
  - const privateKey = bitcoin.HDNode.fromBase58(key.prv).getKey();
  - const privateKeyBuffer = privateKey.d.toBuffer(32);
  - const isCompressed = privateKey.compressed;
  - const prefix = bitcoin.networks.bitcoin.messagePrefix;
  - return bitcoinMessage.sign(message, privateKeyBuffer, isCompressed, prefix);
  + return signMessage(message, bip32.fromBase58(key.prv), bitcoin.networks.bitcoin);
  })
  .call(this)
  .asCallback(callback);
  