std::optional makes a few things a bit neater and less error prone.
However, we still cannot use C++17 (unfortunately), so this commit
adds an implementation of std::optional that we can use right now.
Based on https://github.com/tensorflow/tensorflow/blob/master/tensorflow/core/lib/gtl/optional.h
which seems to be fairly similar to C++17's <optional> and standards
compliant. It's one of the few implementations that handle propagating
type traits like copy constructibility, just like libc++/libstdc++.
