[PATCH] libstdc++: hashing support for chrono value classes (P2592R2)

[Giuseppe D'Angelo]

Hello,

The attached patch implements P2592, adding std::hash specializations 
for std::chrono classes.

One aspect I'm quite unhappy with is the hash combiner I've used. I'm 
not sure if there's some longer-term goal for libstdc++ here -- would 
you prefer to roll something à la Boost.HashCombine / P0814?

Not only it's likely to be cheaper, but it would also be more 
constexpr-friendly, in preparation of constexpr std::hash (P3372).

As usual, any feedback is appreciated :)

Thank you,
-- 
Giuseppe D'Angelo
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