Create bootstrap executor with file I/O

[cevans87]

This executor should be just enough to provide basic file I/O functionality to bootstrap/Basis.

• Owns/manages an ioring
• boostrap/Basis functions submit in the executor ioring
  • openat
  • read
  • write
  • seek
  • close
• bootstrap/Basis receive function receives completed I/O functions
  • openat
  • read
  • write
  • seek
  • close

We're not implementing the actor mailbox yet, we only have one executor, and we also do not have time slice interrupts. Knowing that, we're making the following design decisions.

• We malloc a space for CQEs to be copied to and use the pointer as the SQE user_data. Every time we see a CQE in the completion queue, we copy it to the memory location pointed to by the user_data field.
• We only call io_uring_enter when we try to submit an SQE into a full submission queue, or when we block waiting for a completion.
• We'll construct a global singleton executor and register an atexit handler to clean it up.