controlclient/netmap: write our own b.ConciseDiffFrom(a) function.

This removes the need for go-cmp, which is extremely bloaty so we had
to leave it out of iOS. As a result, we had also left it out of macOS,
and so we didn't print netmap diffs at all on darwin-based platforms.
Oops.

As a bonus, the output format of the new function is way better.

Minor oddity: because I used the dumbest possible diff algorithm, the
sort order is a bit dumb. We print all "removed" lines and then print
all "added" lines, rather than doing the usual diff-like thing of
interspersing them. This probably doesn't matter (maybe it's an
improvement).

control/controlclient/netmap.go

@@ -115,6 +115,34 @@ func (nm *NetworkMap) Concise() string {
 	return buf.String()
 }
 
+func (b *NetworkMap) ConciseDiffFrom(a *NetworkMap) string {
+	out := []string{}
+	ra := strings.Split(a.Concise(), "\n")
+	rb := strings.Split(b.Concise(), "\n")
+
+	ma := map[string]struct{}{}
+	for _, s := range ra {
+		ma[s] = struct{}{}
+	}
+
+	mb := map[string]struct{}{}
+	for _, s := range rb {
+		mb[s] = struct{}{}
+	}
+
+	for _, s := range ra {
+		if _, ok := mb[s]; !ok {
+			out = append(out, "-"+s)
+		}
+	}
+	for _, s := range rb {
+		if _, ok := ma[s]; !ok {
+			out = append(out, "+"+s)
+		}
+	}
+	return strings.Join(out, "\n")
+}
+
 func (nm *NetworkMap) JSON() string {
 	b, err := json.MarshalIndent(*nm, "", "  ")
 	if err != nil {