Type fixes to uart.h (take span)

Change-Id: Icde8dc0c8a7411f7ba876bd364a7911f993d0ec0
diff --git a/y2019/jevois/uart.cc b/y2019/jevois/uart.cc
index e3520a4..0db59a5 100644
--- a/y2019/jevois/uart.cc
+++ b/y2019/jevois/uart.cc
@@ -51,7 +51,7 @@
 }
 
 tl::optional<Frame> UartUnpackToTeensy(
-    const UartToTeensyBuffer &encoded_buffer) {
+    gsl::span<const char> encoded_buffer) {
   std::array<char, uart_to_teensy_size()> buffer;
   if (static_cast<size_t>(
           CobsDecode<uart_to_teensy_size()>(encoded_buffer, &buffer).size()) !=
@@ -138,7 +138,7 @@
 }
 
 tl::optional<CameraCalibration> UartUnpackToCamera(
-    const UartToCameraBuffer &encoded_buffer) {
+    gsl::span<const char> encoded_buffer) {
   std::array<char, uart_to_camera_size()> buffer;
   if (static_cast<size_t>(
           CobsDecode<uart_to_camera_size()>(encoded_buffer, &buffer).size()) !=