Implement `std::sys::theseus`

[tsoutsman]

We need to implement the following files in library/std/src/sys/theseus:

• [x] alloc.rs
• [ ] args.rs Need Rust binary support in Theseus.
• [x] common.rs
  • [x] hashmap_random_keys theseus-os/Theseus#582
• [x] env.rs
• [ ] fs.rs https://github.com/theseus-os/Theseus/pull/639
• [x] io.rs
• [x] locks
  • [x] mutex.rs
  • [x] rwlock.rs
  • [x] condvar.rs
• [ ] net.rs
• [x] os.rs Mostly implemented except for:
  • [x] current_exe
  • [x] join_paths - Theseus doesn't have the concept of a PATH environment variable
• [x] pipe.rs Don't need to implement this.
• [ ] process.rs Needs Theseus standard I/O redesign.
• [x] stdio.rs
• [ ] thread.rs Mostly implemented except for:
  • [ ] Thread::sleep We probably want to wait for theseus-os/Theseus#569 before implementing sleep.
  • [ ] available_parallelism
  • [x] guard
• [x] thread_local_dtor.rs
• [x] thread_local_key.rs
• [x] time.rs theseus-os/Theseus#615

[kevinaboos]

If we do end up needing pipe, we can implement it atop one of our channel implementations.