Add build rules for the GSL

This provides many helpful things, such as span.

Change-Id: I4d2a88b0a7cb93fe36a98f21f4a6874e335e063e
diff --git a/third_party/Catch2/BUILD b/third_party/Catch2/BUILD
new file mode 100644
index 0000000..5dab07c
--- /dev/null
+++ b/third_party/Catch2/BUILD
@@ -0,0 +1,11 @@
+licenses(["notice"])
+
+# The source file was downloaded from:
+# https://github.com/catchorg/Catch2/releases/download/v2.2.3/catch.hpp
+
+cc_library(
+    name = "Catch2",
+    hdrs = ["catch/catch.hpp"],
+    includes = ["."],
+    visibility = ["//visibility:public"],
+)