Auto merge of #36341 - sagebind:thread_id, r=alexcrichton

Add ThreadId for comparing threads

This adds the capability to store and compare threads with the current calling thread via a new struct, `std::thread::ThreadId`. Addresses the need outlined in issue #21507.

This avoids the need to add any special checks to the existing thread structs and does not rely on the system to provide an identifier for a thread, since it seems that this approach is unreliable and undesirable. Instead, this simply uses a lazily-created, thread-local `usize` whose value is copied from a global atomic counter. The code should be simple enough that it should be as much reliable as the `#[thread_local]` attribute it uses (however much that is).

`ThreadId`s can be compared directly for equality and have copy semantics.

Also see these other attempts:
- rust-lang/rust#29457
- rust-lang/rust#29448
- rust-lang/rust#29447

And this in the RFC repo: rust-lang/rfcs#1435

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