Fix UART serialization

Didn't quite think it through before, and the test was buggy too.

Change-Id: Iaf0fa9d95731500850922217bb8614fc57f4cbb6
diff --git a/y2019/jevois/uart.h b/y2019/jevois/uart.h
index 32ca110..8244fdb 100644
--- a/y2019/jevois/uart.h
+++ b/y2019/jevois/uart.h
@@ -13,7 +13,8 @@
 namespace jevois {
 
 constexpr size_t uart_to_teensy_size() {
-  return 3 /* targets */ * (sizeof(float) * 4 /* fields */) + 1 /* age */ +
+  return 1 /* number of targets */ +
+         3 /* targets */ * (sizeof(float) * 4 /* fields */) + 1 /* age */ +
          2 /* CRC-16 */;
 }
 using UartToTeensyBuffer =