core: update *.Contract.* interops

1. Remove GetScript, IsPayable, GetStorageContext.
2. Revert 82319538 related to GetStorageContext.
3. Rename Migrate to Update.
4. Move remaining to System.Contract.*.

Related #1031.

pkg/core/interops_test.go

@@ -32,9 +32,6 @@ func TestUnexpectedNonInterops(t *testing.T) {
 
 	// All of these functions expect an interop item on the stack.
 	funcs := []func(*interop.Context, *vm.VM) error{
-		contractGetScript,
-		contractGetStorageContext,
-		contractIsPayable,
 		storageContextAsReadOnly,
 		storageDelete,
 		storageFind,