interop: move into pkg/interop, replace pkg/vm/api

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.

pkg/interop/engine/engine.go

@@ -0,0 +1,29 @@
+package engine
+
+import "github.com/CityOfZion/neo-go/pkg/interop/transaction"
+
+// Package engine provides function signatures that can be used inside
+// smart contracts that are written in the neo-go framework.
+
+// GetScriptContainer returns the transaction that is in the execution context.
+func GetScriptContainer() transaction.Transaction {
+	return transaction.Transaction{}
+}
+
+// GetExecutingScriptHash returns the script hash of the contract that is
+// currently being executed.
+func GetExecutingScriptHash() []byte {
+	return nil
+}
+
+// GetCallingScriptHash returns the script hash of the contract that started
+// the execution of the current script.
+func GetCallingScriptHash() []byte {
+	return nil
+}
+
+// GetEntryScriptHash returns the script hash of the contract that started the
+// execution from the start.
+func GetEntryScriptHash() []byte {
+	return nil
+}