Add an unstable conversion from thread ID to u64

We see multiple cases inside rustc and ecosystem code where ThreadId is
transmuted to u64, exploiting the underlying detail. This is suboptimal
(can break unexpectedly if we change things in std).

It is unlikely that ThreadId will ever need to be larger than u64 --
creating even 2^32 threads over the course of a program is quite hard,
2^64 is even harder. As such, we do not choose to return a larger sized
type (e.g. u128). If we choose to shrink ThreadId in the future, or
otherwise change its internals, it is likely that a mapping to u64 will
still be applicable (though may become more complex).

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