Use `std::thread::ThreadId` instead of custom thread id

[Noratrieb]

std guarantees them to be unique, allowing us to use them here instead of having to roll our own thread ids.

[mitsuhiko]

This undoes the niche optimization: https://github.com/mitsuhiko/fragile/pull/18

[Noratrieb]

ThreadId uses a NonZeroU64 as well, keeping the niche

[Noratrieb]

It does have the disadvantage of being a little bigger on 32 bit though.

[mitsuhiko]

Oh I missed that. I will reopen this for now but I'm not sure yet what the advantage of reusing the threadid from the stdlib is.

[Noratrieb]

It makes the code a little simpler and avoids the need for having a custom thread_id module. But since it's a pretty simple module there isn't a huge benefit.

[Noratrieb]

Do you think we should do this? I'd resolve the conflicts then. If not, I can close it

[mitsuhiko]

I’m probably in favor of not applying this. It makes the type bigger and I’m not convinced (yet at least) that using the thread ID from the stdlib is necessary. No need to resolve the conflicts but I will leave this open in case my opinion on this changes / gets changed :)