Previously we could generate dynamic appcall with a kludge of
AppCall([]byte{/* 20 zeroes */, realScriptHash, args...)
Now there is a separate function for this.
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.
