SegWit wallet support

This introduces two command line flags (-addresstype and -changetype) which control
the type of addresses/outputs created by the GUI and RPCs. Certain RPCs allow
overriding these (`getnewaddress` and `getrawchangeaddress`). Supported types
are "legacy" (P2PKH and P2SH-multisig), "p2sh-segwit" (P2SH-P2WPKH and P2SH-P2WSH-multisig),
and "bech32" (P2WPKH and P2WSH-multisig).

A few utility functions are added to the wallet to construct different address type
and to add the necessary entries to the wallet file to be compatible with earlier
versions (see `CWallet::LearnRelatedScripts`, `GetDestinationForKey`,
`GetAllDestinationsForKey`, `CWallet::AddAndGetDestinationForScript`).

test/functional/bip68-sequence.py

@@ -362,9 +362,10 @@ class BIP68Test(BitcoinTestFramework):
         block.vtx.extend([tx1, tx2, tx3])
         block.hashMerkleRoot = block.calc_merkle_root()
         block.rehash()
+        add_witness_commitment(block)
         block.solve()
 
-        self.nodes[0].submitblock(ToHex(block))
+        self.nodes[0].submitblock(bytes_to_hex_str(block.serialize(True)))
         assert_equal(self.nodes[0].getbestblockhash(), block.hash)
 
     def activateCSV(self):