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/header/header.go

@@ -0,0 +1,47 @@
+package header
+
+// Package header provides function signatures that can be used inside
+// smart contracts that are written in the neo-go framework.
+
+// Header stubs a NEO block header type.
+type Header struct{}
+
+// GetIndex returns the index of the given header.
+func GetIndex(h Header) int {
+	return 0
+}
+
+// GetHash returns the hash of the given header.
+func GetHash(h Header) []byte {
+	return nil
+}
+
+// GetPrevHash returns the previous hash of the given header.
+func GetPrevHash(h Header) []byte {
+	return nil
+}
+
+// GetTimestamp returns the timestamp of the given header.
+func GetTimestamp(h Header) int {
+	return 0
+}
+
+// GetVersion returns the version of the given header.
+func GetVersion(h Header) int {
+	return 0
+}
+
+// GetMerkleRoot returns the merkle root of the given header.
+func GetMerkleRoot(h Header) []byte {
+	return nil
+}
+
+// GetConsensusData returns the consensus data of the given header.
+func GetConsensusData(h Header) int {
+	return 0
+}
+
+// GetNextConsensus returns the next consensus of the given header.
+func GetNextConsensus(h Header) []byte {
+	return nil
+}