std: split up the `thread` module
Almost all functionality in `std::thread` is currently implemented in `thread/mod.rs`, resulting in a *huge* file with more than 2000 lines and multiple, interoperating `unsafe` sections. This PR splits the file up into multiple different private modules, each implementing mostly independent parts of the functionality. The only remaining `unsafe` interplay is that of the `lifecycle` and `scope` modules, the `spawn_scoped` implementation relies on the live thread count being updated correctly by the `lifecycle` module.
This PR contains no functional changes and only moves code around for the most part, with a few notable exceptions:
* `with_current_name` is moved to the already existing `current` module and now uses the `name` method instead of calculating the name from private fields. The old code was just a reimplementation of that method anyway.
* The private `JoinInner` type used to implement both join handles now has some more methods (`is_finished`, `thread` and the `AsInner`/`IntoInner` implementations) to avoid having to expose private fields and their invariants.
* The private `spawn_unchecked_` (note the underscore) method of `Builder` is now a freestanding function in `lifecycle`.
The rest of the changes are just visibility annotations.
I realise this PR ended up quite large – let me know if there is anyway I can aid the review process.
Edit: I've simplified the diff by adding an intermediate commit that creates all the new files by duplicating `mod.rs`. The actual changes in the second commit thus appear to delete the non-relevant parts from the respective file.
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