lnwire: add new 32-byte persistent/pending channel ID's

This commit does to things: moves the prior ShortChannelID struct into
a new short_channel_id.go file, and also implements the new ChannelID’s
currently used within he specification.

These new ID’s are 32-bytes in length and used during initial channel
funding as well as during normal channel updates. During initial
channel funding, the ID is to be a random 32-byte string, while once
normal channel operation has began, the ID is to be (txid XOR index),
where index is the index of the funding outpoint.

lnwire/short_channel_id_test.go

@@ -0,0 +1,39 @@
+package lnwire
+
+import (
+	"reflect"
+	"testing"
+
+	"github.com/davecgh/go-spew/spew"
+)
+
+func TestShortChannelIDEncoding(t *testing.T) {
+	var testCases = []ShortChannelID{
+		{
+			BlockHeight: (1 << 24) - 1,
+			TxIndex:     (1 << 24) - 1,
+			TxPosition:  (1 << 16) - 1,
+		},
+		{
+			BlockHeight: 2304934,
+			TxIndex:     2345,
+			TxPosition:  5,
+		},
+		{
+			BlockHeight: 9304934,
+			TxIndex:     2345,
+			TxPosition:  5233,
+		},
+	}
+
+	for _, testCase := range testCases {
+		chanInt := testCase.ToUint64()
+
+		newChanID := NewShortChanIDFromInt(chanInt)
+
+		if !reflect.DeepEqual(testCase, newChanID) {
+			t.Fatalf("chan ID's don't match: expected %v got %v",
+				spew.Sdump(testCase), spew.Sdump(newChanID))
+		}
+	}
+}