Barrier parity waiting and algorithm tweaks

[ogiroux]

Adds the ability to wait on a barrier when having only retained phase parity information (from domain knowledge, it isn't recoverable from the barrier object or its tokens), like so:

barrier<> b (1);
(void)b.arrive(); // discard the nodiscard return
barrier_wait_for_parity(&b, false);

Also changed the algorithm so that some b.arrive(); can be equivalent to just a fetch_add on its constituent atomic. These are available with the __expect_extra_arrive and __extra_arrive members.

The parity waiting part isn't ABI breaking, but the algorithm tweak is ABI breaking because it halves the value of ::max(), to half of int max (which is somewhat theoretical an issue given how large this number is). I didn't add ABI handling code.

[ogiroux]

I amended the description above after reducing the ABI exposure.

[brycelelbach]

Closing in favor of #111