Add SegWit support

[ilyavf]

Add an option for building SegWit transactions.

Sources:

• Main article: "Segregated Witness Wallet Development Guide" https://bitcoincore.org/en/segwit_wallet_dev/
• SegWit info: https://github.com/bitcoinbook/bitcoinbook/blob/develop/ch07.asciidoc#segregated-witness
• Tool for tx check: http://n.bitcoin.ninja/checktx?txid=d869f854e1f8788bcff294cc83b280942a8c728de71eb709a2c29d10bfe21b7c
• Testnet SegNet: https://bitcoincore.org/en/2016/01/21/launch_segwit_testnet/

Details:

• SegWit outputs are constructed so that older systems that are not segwit-aware can still validate them. To an old wallet or node, a SegWit output looks like an output that anyone can spend.
• Newer wallets and mining nodes, however, see the Segregated Witness output and expect to find a valid witness for it in the transaction’s witness data.
• Pay-to-Witness-Public-Key-Hash (P2WPKH) https://github.com/bitcoinbook/bitcoinbook/blob/develop/ch07.asciidoc#pay-to-witness-public-key-hash-p2wpkh Examples:

  // P2PKH output script
  DUP HASH160 ab68025513c3dbd2f7b92a94e0581f5d50f654e7 EQUALVERIFY CHECKSIG

  // P2WPKH output script (includes: a witness version 0)
  0 ab68025513c3dbd2f7b92a94e0581f5d50f654e7

  // To spend the Segregated Witness output:
  [...]
  "vin" : [{
      "txid": "0627052b6f28912f2703066a912ea577f2ce4da4caa5a5fbd8a57286c345c2f2",
      "vout": 0,
      "scriptSig": ""
  }]
  [...]
  "witness": "<Bob’s witness data>"
  [...]

• Pay-to-Witness-Script-Hash (P2WSH) The second type of witness program corresponds to a Pay-to-Script-Hash (P2SH) script. Examples:

  // 1. P2SH
  // Includes 20-byte RIPEMD160(SHA256(script)) hash:
  HASH160 54c557e07dde5bb6cb791c7a540e0a4796f5e97e EQUAL
  
  // P2SH output being spent:
  "vout": 0,
       "scriptSig": “<SigA> <SigB> <2 PubA PubB PubC PubD PubE 5 CHECKMULTISIG>”,
  ]
  
  // 2. P2WSH output script
  // Includes a witness version (0) and 32-byte SHA256 hash of the redeem script.
  0 a9b7b38d972cabc7961dbfbcb841ad4508d133c47ba87457b4a0e8aae86dbb89
  
  // P2WSH output being spent with separate witness data
  "vout": 0,
       "scriptSig": “”,
  ]
  [...]
  "witness": "<SigA> <SigB> <2 PubA PubB PubC PubD PubE 5 CHECKMULTISIG>"

• Wallet construction of P2WPKH
  • It is extremely important to note that P2WPKH should only be created by the payee (recipient) and not converted by the sender from a known public key, P2PKH script, or address. Sender does not know if the recipient has SegWit support.
  • P2WPKH outputs must be constructed from the hash of a compressed public key.

Transaction:

• If all txins in a transaction are not associated with any witness data, the transaction MUST be serialized in the original transaction format.
• Under segwit, each transaction will have 2 IDs.

[ilyavf]

API:

• add option to choose what type of script should be built:
  • P2WPKH (pure SegWit);
  • P2SH (embed SegWit for older nodes).
• distinguish between SegWit addresses (Bech32) and Base58Check.

  // buildTx :: (Tx, Options) -> Buffer
  const tx = {
  vin: [{
    txid: '2d7a9f0534ddac231ef1978bda388791c32321f7e14e18e7df3bbed261615f54',
    index: 1,
    script: '',   // ?
    segwit: ''    // ?
  }],
  vout: [{
  value: 100000000,
  address: 'tb1qw508d6qejxtdg4y5r3zarvary0c5xw7kxpjzsx'
  segwit: 'P2WPKH'     // << define the output script type here.
  }, { ... }]
  }
  buildTx(tx)

Cases:

1. VIN: regular UTXO; VOUT: P2SH-P2WPKH; regular transaction structure.

Steps:

1. Create P2SH-P2WPKH address