Sdk2 kvpair (#102)

* Canonical KVPair in common
* Simplify common/Bytes to just hex encode

common/bytes.go

@@ -0,0 +1,53 @@
+package common
+
+import (
+	"encoding/hex"
+	"fmt"
+	"strings"
+)
+
+// The main purpose of Bytes is to enable HEX-encoding for json/encoding.
+type Bytes []byte
+
+// Marshal needed for protobuf compatibility
+func (b Bytes) Marshal() ([]byte, error) {
+	return b, nil
+}
+
+// Unmarshal needed for protobuf compatibility
+func (b *Bytes) Unmarshal(data []byte) error {
+	*b = data
+	return nil
+}
+
+// This is the point of Bytes.
+func (b Bytes) MarshalJSON() ([]byte, error) {
+	s := strings.ToUpper(hex.EncodeToString(b))
+	jb := make([]byte, len(s)+2)
+	jb[0] = '"'
+	copy(jb[1:], []byte(s))
+	jb[1] = '"'
+	return jb, nil
+}
+
+// This is the point of Bytes.
+func (b *Bytes) UnmarshalJSON(data []byte) error {
+	if len(data) < 2 || data[0] != '"' || data[len(data)-1] != '"' {
+		return fmt.Errorf("Invalid hex string: %s", data)
+	}
+	bytes, err := hex.DecodeString(string(data[1 : len(data)-1]))
+	if err != nil {
+		return err
+	}
+	*b = bytes
+	return nil
+}
+
+// Allow it to fulfill various interfaces in light-client, etc...
+func (b Bytes) Bytes() []byte {
+	return b
+}
+
+func (b Bytes) String() string {
+	return strings.ToUpper(hex.EncodeToString(b))
+}