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/attribute/attribute.go

@@ -0,0 +1,17 @@
+package attribute
+
+// Package attribute provides function signatures that can be used inside
+// smart contracts that are written in the neo-go framework.
+
+// Attribute stubs a NEO transaction attribute type.
+type Attribute struct{}
+
+// GetUsage returns the usage of the given attribute.
+func GetUsage(attr Attribute) byte {
+	return 0x00
+}
+
+// GetData returns the data of the given attribute.
+func GetData(attr Attribute) []byte {
+	return nil
+}