Refactor client into separate files

Add a main loop, move I/O code into io.go, move configuration into config.go.
Use shared.Decode24BitLength instead of manually decoding block lengths.
Fix response block decoding and checksum validation.
Add constants for commonly used byte values and use these in the signer and
the client.

datastructures/common.go

@@ -6,6 +6,15 @@ type Action uint8
 
 const ActionNul = Action(0)
 
+func (a Action) String() string {
+	switch a {
+	case ActionNul:
+		return "NUL"
+	default:
+		return "unknown"
+	}
+}
+
 func encode24BitLength(data []byte) []byte {
 	lengthBytes := make([]byte, 4)
 	binary.BigEndian.PutUint32(lengthBytes, uint32(len(data)))
@@ -13,7 +22,7 @@ func encode24BitLength(data []byte) []byte {
 }
 
 // calculate length from 24 bits of data in network byte order
-func decode24BitLength(bytes []byte) int {
+func Decode24BitLength(bytes []byte) int {
 	return int(binary.BigEndian.Uint32([]byte{0x0, bytes[0], bytes[1], bytes[2]}))
 }