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.