Feature: add ssr support (#805)

* Refactor ssr stream cipher to expose iv and key

References:
https://github.com/Dreamacro/go-shadowsocks2
https://github.com/sh4d0wfiend/go-shadowsocksr2

* Implement ssr obfs

Reference:
https://github.com/mzz2017/shadowsocksR

* Implement ssr protocol

References:
https://github.com/mzz2017/shadowsocksR
https://github.com/shadowsocksRb/shadowsocksr-libev
https://github.com/shadowsocksr-backup/shadowsocksr

component/ssr/protocol/packet.go

@@ -0,0 +1,42 @@
+package protocol
+
+import (
+	"net"
+
+	"github.com/Dreamacro/clash/common/pool"
+)
+
+// NewPacketConn returns a net.NewPacketConn with protocol decoding/encoding
+func NewPacketConn(pc net.PacketConn, p Protocol) net.PacketConn {
+	return &PacketConn{PacketConn: pc, Protocol: p.initForConn(nil)}
+}
+
+// PacketConn represents a protocol packet connection
+type PacketConn struct {
+	net.PacketConn
+	Protocol
+}
+
+func (c *PacketConn) WriteTo(b []byte, addr net.Addr) (int, error) {
+	buf := pool.Get(pool.RelayBufferSize)
+	defer pool.Put(buf)
+	buf, err := c.EncodePacket(b)
+	if err != nil {
+		return 0, err
+	}
+	_, err = c.PacketConn.WriteTo(buf, addr)
+	return len(b), err
+}
+
+func (c *PacketConn) ReadFrom(b []byte) (int, net.Addr, error) {
+	n, addr, err := c.PacketConn.ReadFrom(b)
+	if err != nil {
+		return n, addr, err
+	}
+	bb, length, err := c.DecodePacket(b[:n])
+	if err != nil {
+		return n, addr, err
+	}
+	copy(b, bb)
+	return length, addr, err
+}