Reduce gsl::span usage so we can remove it

We already have absl::Span, no need to add more animals to the zoo.
This should let us delete 2 libraries out of third_party.

While we are here, the teensy code for 2019 wasn't being built.  Fix
that.

Change-Id: I247750016926122240bc76e6194cf2ddfc15e9c2
Signed-off-by: Austin Schuh <austin.linux@gmail.com>
diff --git a/y2019/vision/BUILD b/y2019/vision/BUILD
index e005e70..87ce17e 100644
--- a/y2019/vision/BUILD
+++ b/y2019/vision/BUILD
@@ -15,7 +15,6 @@
         "constants_formatting.cc",
     ],
     hdrs = ["constants.h"],
-    target_compatible_with = ["@platforms//os:linux"],
     visibility = ["//visibility:public"],
 )
 
diff --git a/y2019/vision/debug_serial.cc b/y2019/vision/debug_serial.cc
index d1c70c2..a658bd7 100644
--- a/y2019/vision/debug_serial.cc
+++ b/y2019/vision/debug_serial.cc
@@ -36,7 +36,7 @@
       char data[kBufferSize];
       ssize_t n = read(itsDev, &data[0], kBufferSize);
       if (n >= 1) {
-        cobs.ParseData(gsl::span<const char>(&data[0], n));
+        cobs.ParseData(absl::Span<const char>(&data[0], n));
         auto packet = cobs.received_packet();
         if (!packet.empty()) {
           // One we read data from the serial, Teensy code will return an
diff --git a/y2019/vision/target_sender.cc b/y2019/vision/target_sender.cc
index b7fed9a..4aa4b54 100644
--- a/y2019/vision/target_sender.cc
+++ b/y2019/vision/target_sender.cc
@@ -316,7 +316,7 @@
       char data[kBufferSize];
       ssize_t n = read(itsDev, &data[0], kBufferSize);
       if (n >= 1) {
-        cobs.ParseData(gsl::span<const char>(&data[0], n));
+        cobs.ParseData(absl::Span<const char>(&data[0], n));
         auto packet = cobs.received_packet();
         if (!packet.empty()) {
           auto calibration_question =