Resulting item can't have more than MaxStackSize elements. Technically this
limits to MaxStackSize cloned elements but it's considered to be enough to
mitigate the issue (the next size check is going to happen during push to the
stack). See neo-project/neo#2534, thanks @vang1ong7ang.
Most often we only need to read them and it doesn't require copying. Make an
explicit copy (and copy only things we need!) where needed.
After the recent neo-vm tests update our vm package testing time jumped to
~12s, with this change it's now more like ~8s.
Inspired by neo-project/neo-vm#352. We can't directly compare slices, so we're
better optimize things we already have. At the same time this code would
behave a bit different if A is to call B and then B is call A and then some
pointer from the first A invocation is to be compared with a pointer from the
second A invocation. Not sure it really matters.
