A future that does not carry any data (future<>) and its sibling (promise<>)
are heavily used in the code. We can optimize them by overlaying the
future's payload, which in this case can only be an std::exception_ptr,
with the future state, as a pointer and an enum have disjoint values.
This of course depends on std::exception_ptr being implemented as a pointer,
but as it happens, it is.
With this, sizeof(future<>) is reduced from 24 bytes to 16 bytes.
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