native: add Ledger contract, fix #1696

But don't change the way we process/store transactions and blocks. Effectively
it's just an interface for smart contracts that replaces old syscalls.

Transaction definition is moved temporarily to runtime package and Block
definition is removed (till we solve #1691 properly).

pkg/interop/runtime/engine.go

@@ -2,15 +2,38 @@ package runtime
 
 import (
 	"github.com/nspcc-dev/neo-go/pkg/interop"
-	"github.com/nspcc-dev/neo-go/pkg/interop/blockchain"
 )
 
+// Transaction represents a NEO transaction. It's similar to Transaction class
+// in Neo .net framework.
+type Transaction struct {
+	// Hash represents the hash (256 bit BE value in a 32 byte slice) of the
+	// given transaction (which also is its ID).
+	Hash interop.Hash256
+	// Version represents the transaction version.
+	Version int
+	// Nonce is a random number to avoid hash collision.
+	Nonce int
+	// Sender represents the sender (160 bit BE value in a 20 byte slice) of the
+	// given Transaction.
+	Sender interop.Hash160
+	// SysFee represents fee to be burned.
+	SysFee int
+	// NetFee represents fee to be distributed to consensus nodes.
+	NetFee int
+	// ValidUntilBlock is the maximum blockchain height exceeding which
+	// transaction should fail verification.
+	ValidUntilBlock int
+	// Script represents code to run in NeoVM for this transaction.
+	Script []byte
+}
+
 // GetScriptContainer returns the transaction that initially triggered current
 // execution context. It never changes in a single execution, no matter how deep
 // this execution goes. This function uses
 // `System.Runtime.GetScriptContainer` syscall.
-func GetScriptContainer() *blockchain.Transaction {
-	return &blockchain.Transaction{}
+func GetScriptContainer() *Transaction {
+	return &Transaction{}
 }
 
 // GetExecutingScriptHash returns script hash (160 bit in BE form represented