add package swarm with Address type

pkg/swarm/swarm.go

+// Copyright 2020 The Swarm Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package swarm contains most basic and general Swarm concepts.
+package swarm
+
+import "encoding/hex"
+
+// Address represents an address in Swarm metric space of
+// Node and Chunk addresses.
+type Address []byte
+
+// NewAddress returns an Address from a hex-encoded string representation.
+func NewAddress(s string) (Address, error) {
+	return hex.DecodeString(s)
+}
+
+// String returns a hex-encoded representation of the Address.
+func (a Address) String() string {
+	return hex.EncodeToString(a)
+}