types: Add StringInt64Map

A map[string]int64 that is able to properly encode and decode according
to the format being used by the steemd encoder.

types/map.go

@@ -0,0 +1,57 @@
+package types
+
+import (
+	"encoding/json"
+	"errors"
+)
+
+type StringInt64Map map[string]int64
+
+func (m StringInt64Map) MarshalJSON() ([]byte, error) {
+	xs := make([]interface{}, len(m))
+	for k, v := range m {
+		xs = append(xs, []interface{}{k, v})
+	}
+	return json.Marshal(xs)
+}
+
+func (m *StringInt64Map) UnmarshalJSON(data []byte) error {
+	var xs [][]interface{}
+	if err := json.Unmarshal(data, &xs); err != nil {
+		return err
+	}
+
+	var invalid bool
+	mp := make(map[string]int64, len(xs))
+	for _, kv := range xs {
+		if len(kv) != 2 {
+			invalid = true
+			break
+		}
+
+		k, ok := kv[0].(string)
+		if !ok {
+			invalid = true
+			break
+		}
+
+		var v int64
+		switch t := kv[1].(type) {
+		case float64:
+			v = int64(t)
+		case int64:
+			v = t
+		default:
+			invalid = true
+			break
+		}
+
+		mp[k] = v
+	}
+	if invalid {
+		return errors.New("invalid map encoding")
+	}
+
+	*m = mp
+	return nil
+}