core/state: track all accounts in canon state

This change introduces a global, per-state cache that keeps account data
in the canon state. Thanks to @karalabe for lots of fixes.

core/vm/environment.go

@@ -94,6 +94,7 @@ type Database interface {
 	GetNonce(common.Address) uint64
 	SetNonce(common.Address, uint64)
 
+	GetCodeSize(common.Address) int
 	GetCode(common.Address) []byte
 	SetCode(common.Address, []byte)
 

core/vm/instructions.go

@@ -363,7 +363,7 @@ func opCalldataCopy(instr instruction, pc *uint64, env Environment, contract *Co
 
 func opExtCodeSize(instr instruction, pc *uint64, env Environment, contract *Contract, memory *Memory, stack *Stack) {
 	addr := common.BigToAddress(stack.pop())
-	l := big.NewInt(int64(len(env.Db().GetCode(addr))))
+	l := big.NewInt(int64(env.Db().GetCodeSize(addr)))
 	stack.push(l)
 }