Improve PACK/UNPACK opcodes about neo-vm

[shargon]

Can we use UNPACK ?

[igormcoelho]

Very goooooood Shargon! Now it's better... we just use CONVERT and then use UNPACK.
But that will work? Because unpack puts a lot of elements in stack... we would need to somehow unpack [0x01,0x02,0x03] into 0x010203. But you gave me an idea, perhaps we could reuse PACK, if a bytearray is on stack then it just converts it to array, byte to byte:
https://github.com/neo-project/neo-vm/blob/master/src/neo-vm/ExecutionEngine.cs#L699

The thing is, we would need to check the type, to make sure it's a biginteger and not a byte array, and since many codes do PUSH1 - PACK, is this "1" a bytearray with a single element, or number 1?

[igormcoelho]

Can we use UNPACK ?

Now I got your point Shargon!! I think UNPACK is good to work as a conversor from ByteArray to Array:

neo-vm/src/neo-vm/ExecutionEngine.cs

Lines 713 to 719 in 94a91ae

	StackItem item = context.EvaluationStack.Pop();
	if (item is VMArray array)
	{
	for (int i = array.Count - 1; i >= 0; i--)
	context.EvaluationStack.Push(array[i]);
	context.EvaluationStack.Push(array.Count);
	}

So, it could be:

PUSH10 ababababab...
UNPACK # (new UNPACK function) converts byte[] to Array (of byte) on stack [ab,ab,ab...]
DUP # duplicates Array on stack
REVERSE  # reverse and drop array
???  # convert Array back to byte[] on stack

Now we need to "really unpack" and apply several concats here, to merge them in a single byte array.

======

GOT IT!! Now we use PACK again hahaah but PACK receives an integer usually, if it receives an Array, then it know it has to merge to a single byte[] ;)

PUSH10 ababababab...
UNPACK # (new UNPACK functionality) converts byte[] to Array (of byte) on stack [ab,ab,ab...]
DUP # duplicates Array on stack
REVERSE  # reverse and drop array
PACK  # (new PACK functionality) convert Array back to byte[] on stack

No need for CONVERT xD

[igormcoelho]

Now I think this breaks semantics, and we have much better options for Neo3.