bencode: add missing error handling

In addition, this PR attempts to simplify some functions according to
the output of `gocyclo -n 15`.

frontend/http/bencode/bencode.go

@@ -2,6 +2,11 @@
 // type assertion over reflection for performance.
 package bencode
 
+import "bytes"
+
+// Enforce that Dict implements the Marshaler interface.
+var _ Marshaler = Dict{}
+
 // Dict represents a bencode dictionary.
 type Dict map[string]interface{}
 
@@ -10,9 +15,26 @@ func NewDict() Dict {
 	return make(Dict)
 }
 
+// MarshalBencode implements the Marshaler interface for Dict.
+func (d Dict) MarshalBencode() ([]byte, error) {
+	var buf bytes.Buffer
+	err := marshalMap(&buf, map[string]interface{}(d))
+	return buf.Bytes(), err
+}
+
+// Enforce that List implements the Marshaler interface.
+var _ Marshaler = List{}
+
 // List represents a bencode list.
 type List []interface{}
 
+// MarshalBencode implements the Marshaler interface for List.
+func (l List) MarshalBencode() ([]byte, error) {
+	var buf bytes.Buffer
+	err := marshalList(&buf, []interface{}(l))
+	return buf.Bytes(), err
+}
+
 // NewList allocates the memory for a List.
 func NewList() List {
 	return make(List, 0)