Part of #1055.
We should check contract scripthash against the one provided in manifest
and manifest groups. We shouldn't put on stack anything after return.
And ofcourse, we mast not destroy the old contract at the end, as
`contractDestroy` removes all storage items associated with the
old contract ID (which equals to the new contract ID). We just remove
old contract state - it's enough.
1. Remove GetScript, IsPayable, GetStorageContext.
2. Revert 82319538 related to GetStorageContext.
3. Rename Migrate to Update.
4. Move remaining to System.Contract.*.
Related #1031.
neo-storm has developed more wrappers for syscall APIs, so they can and should
be used as a drop-in replacement for pkg/vm/api. Moving it out of vm, as it's
not exactly related to the VM itself.
