The changes come from the `functions` branch, where I'm using
these structures a lot.
This also includes a new `BLI::Optional<T>` type, which is similar
to `std::Optional<T>` which can be used when Blender starts using
C++17.
These two data structures reference strings somewhere in memory.
They do not own the referenced string. The string is considered
const.
A string referenced by StringRefNull can be expected to be
null-terminated. That is not the case for StringRef.
This commit is a continuation of rB369d5e8ad2bb7c2.
