Implement `std::sys::theseus`

[tsoutsman]

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

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

[kevinaboos]

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