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/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 =