Tracking Issue for restructuring `std::sys`

[joboet]

On Wednesday, the libs-team discussed reorganizing the platform-specific modules inside std, namely sys and sys_common into a new structure. These modules will be merged into one sys module with a feature-first organization:

• the submodules of sys, named after the feature they implement, contain all the implementations of the different platforms and provide a uniform interface that the rest of std may depend on. E.g. sys::thread_local, sys::fs or sys::thread_parking.
• sys::pal contains platform-specific generic API wrappers like FileDesc cvt (UNIX) used to implement these features. E.g. sys::pal::unix, sys::pal::windows or (perhaps) sys::pal::apple.

The rest of std, modulo std::os, should now only depend on sys::feature_name and not sys::pal. The implementations themselves may however depend on other parts of std as necessary.

Tasks

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1. Move platform modules into sys::pal #117285
   A-meta A-testsuite S-waiting-on-bors T-bootstrap T-compiler T-libs merged-by-bors +7
   
2. Move personality implementation out of PAL #119935
   A-testsuite S-waiting-on-bors T-bootstrap T-libs +4
   
3. Move OS String implementation into sys #119996
   O-SGX O-hermit O-solid O-unix O-wasi O-wasm O-windows S-waiting-on-bors T-libs +9
   
4. Move cmath into sys #120109
   O-SGX O-hermit O-solid O-unix O-wasi O-wasm O-windows S-waiting-on-bors T-libs +9
   
5. Move path implementations into sys #120776
   O-SGX O-hermit O-solid O-unix O-wasi O-wasm O-windows S-waiting-on-bors T-libs +9
   
6. Move locks to sys #121177
   A-meta O-SGX O-hermit O-solid O-unix O-wasi O-wasm O-windows S-waiting-on-bors T-libs merged-by-bors +11
   
7. Move thread local implementation to sys #121758
   S-waiting-on-bors T-libs +2
   
8. Move Once implementations to sys #122386
   A-meta O-wasi O-wasm S-waiting-on-bors T-libs +5
   
9. Remove sys_common::thread #123807
   S-waiting-on-bors T-libs +2
   
10. Move thread parking to sys::sync #124159
    O-SGX O-unix O-wasi O-wasm O-windows S-waiting-on-bors T-libs +7
    
11. std: refactor the TLS implementation #126523
    A-thread-locals O-SGX O-hermit O-itron O-solid O-unix O-wasi O-wasm O-windows S-waiting-on-bors T-libs merged-by-bors +12
    
12. std: move sys_common::backtrace to sys #126546
    S-waiting-on-bors T-libs +2
    
13. std: move allocators to sys #128134
    O-SGX O-hermit O-solid O-unix O-wasi O-wasm O-windows S-waiting-on-bors T-libs merged-by-bors +10
    
14. Move remaining features from sys_common and sys::pal into sys

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