add support for ZCash

Implement block deserialization, tx deserialization/serialization
and input signing for ZCash and generalize those functions in the
client and server btc packages.

Implemenation notes
  1. zcashd does not support encrypted wallets. No passwords allowed.
  2. After starting the harness, it takes a few minutes for beta to
     get caught up.
  3. zcashd can take a very long time to get it's fee estimates primed.

client/asset/bch/bch.go

@@ -170,28 +170,14 @@ func NewWallet(cfg *asset.WalletConfig, logger dex.Logger, network dex.Network)
 		// https://gist.github.com/markblundeberg/a3aba3c9d610e59c3c49199f697bc38b#making-unmalleable-smart-contracts
 		// https://github.com/bitcoin/bips/blob/master/bip-0062.mediawiki
 		NonSegwitSigner: rawTxInSigner,
-		// The old allowHighFees bool argument to sendrawtransaction.
-		ArglessChangeAddrRPC: true,
+		OmitAddressType: true,
 		// Bitcoin Cash uses estimatefee instead of estimatesmartfee, and even
 		// then, they modified it from the old Bitcoin Core estimatefee by
 		// removing the confirmation target argument.
 		FeeEstimator: estimateFee,
 	}
 
-	xcWallet, err := btc.BTCCloneWallet(cloneCFG)
-	if err != nil {
-		return nil, err
-	}
-
-	return &BCHWallet{
-		ExchangeWalletFullNode: xcWallet,
-	}, nil
-}
-
-// BCHWallet embeds btc.ExchangeWalletFullNode, but re-implements a couple of
-// methods to perform on-the-fly address translation.
-type BCHWallet struct {
-	*btc.ExchangeWalletFullNode
+	return btc.BTCCloneWallet(cloneCFG)
 }
 
 // rawTxSigner signs the transaction using Bitcoin Cash's custom signature
@@ -242,7 +228,7 @@ func translateTx(btcTx *wire.MsgTx) (*bchwire.MsgTx, error) {
 		return nil, err
 	}
 
-	bchTx := bchwire.NewMsgTx(bchwire.TxVersion)
+	bchTx := new(bchwire.MsgTx)
 	err = bchTx.Deserialize(bytes.NewBuffer(txB))
 	if err != nil {
 		return nil, err