Adds Abseil Stringify support to nda::interval.

[jiawen]

• In the Bazel version of the build, optionally pulls in abseil-cpp to add preliminary support for AbslStringify() (on nda::interval only).

[jiawen]

@dsharlet Why can't I add @fbleibel-g as a reviewer?

[jiawen]

I think this is missing a new header?

I really despise git at this point.

[jiawen]

I need to change Makefile (or move files around) to ignore test/absl.cpp

[dsharlet]

I need to change Makefile (or move files around) to ignore test/absl.cpp

Maybe there should be a separate folder that contains all of the absl dependent code in it? Maybe that part wouldn't even have a Makefile, if we expect bazel and absl usage to go together?

[jiawen]

I need to change Makefile (or move files around) to ignore test/absl.cpp

Maybe there should be a separate folder that contains all of the absl dependent code in it? Maybe that part wouldn't even have a Makefile, if we expect bazel and absl usage to go together?

I like that. I'll just call it absl and call it absl.h. Then in addition to AbslStringify, I'm tempted to try AbslHashValue (https://abseil.io/docs/cpp/guides/hash)

[dsharlet]

I added @fbleibel-g as a collaborator, maybe that’s what is needed to add him as a reviewer