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.
