These templates are instantiated whenever `<__chrono/duration.h>` is
included, making these calculations quite costly. I also don't think
that folding the calculations decreases readability here (IMO it's
actually easier to read now), so I don't think there is a reason we
shouldn't do this.
`<__chrono/duration.h>` is currently (transitively) included in at least
(I stopped checking) `<algorithm>`, `<atomic>`, `<barrier>`, `<chrono>`,
`<condition_variable>`, `<future>`, `<iomanip>`, `<ios>`, `<iostream>`,
`<istream>`, `<latch>`, `<locale>`, `<mutex>`, `<ostream>`,
`<semaphore>`, `<shared_mutex>`, `<syncstream>` and `<thread>`
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