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.

• Move platform modules into sys::pal #117285
• Move personality implementation out of PAL #119935
• Move OS String implementation into sys #119996
• Move cmath into sys #120109
• Move path implementations into sys #120776
• Move locks to sys #121177
• Move thread local implementation to sys #121758
• Move Once implementations to sys #122386
• Remove sys_common::thread #123807
• Move thread parking to sys::sync #124159
• std: refactor the TLS implementation #126523
• std: move sys_common::backtrace to sys #126546
• std: move allocators to sys #128134
• std: move io module out of pal, get rid of sys_common::io #135696
• std: move network code into sys #136449
• Move fs into sys #137650
• std: move stdio to sys #136780
• std: move process implementations to sys #136929
• std: get rid of sys_common::process #140393
• Move fd into std::sys #139092
• Move args into std::sys #139710
• Move pal::env to std::sys::env_consts #139868
• Move sys::pal::os::Env into sys::env #140143
• Move remaining features from sys_common and sys::pal into sys

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