Optimize writeVarString.

Before:

BenchmarkWriteVarStr4    1000000              1114 ns/op
BenchmarkWriteVarStr10   1000000              1352 ns/op

After:

BenchmarkWriteVarStr4    5000000               291 ns/op
BenchmarkWriteVarStr10  10000000               248 ns/op

This is part ef the ongoing effort to optimize serialization as noted in
conformal/btcd#27.

common.go

@@ -181,7 +181,7 @@ func writeVarString(w io.Writer, pver uint32, str string) error {
 	if err != nil {
 		return err
 	}
-	err = writeElement(w, []byte(str))
+	_, err = w.Write([]byte(str))
 	if err != nil {
 		return err
 	}