An interval object stores five booleans: start()->is_inclusive(),
a boolean since start() itself is an std::optional, two more for
end(), and is_singular(). Due to bad packing, these five booleans
occupy 8 bytes each, for a total of 40 bytes.
Re-pack the interval class by storing those booleans explicitly
close by. Since we lose std::optional's ability to store
a maybe-constructed object, we re-implement it using anonymous
unions and therefore have to implement the 5 special methods.
This helps saves space when vectors of intervals are used, as
seen in #3335 for example.
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