Generally hash functions shouldn't throw exceptions. It's also a
requirement for the standard library-provided hash functions to not
throw exceptions.
An exception to this rule is made for user-defined specializations,
however we can just be consistent with the standard library on this to
allow it to play nicer with it.
While we're at it, we can also make the std::less specializations
noexcpet as well, since they also can't throw.
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