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/aos/util/BUILD b/aos/util/BUILD
index a612438..d75d421 100644
--- a/aos/util/BUILD
+++ b/aos/util/BUILD
@@ -7,7 +7,7 @@
     ],
     visibility = ["//visibility:public"],
     deps = [
-        "//third_party/GSL",
+        "@com_google_absl//absl/types:span",
     ],
 )