Implement the `Sim` Backend

[ammrat13]

This should take advantage of the capabilities of the simulator. This includes the already implemented Trap capability, but it also includes:

• Clockmmio for getting the current system time from hardware
  • gettimeofday
  • times
• Rngmmio for getting entropy from hardware
  • getentropy
• Virtualfs for reading files from the host
  • open
  • close
  • read
  • write
  • lseek
  • stat
  • fstat
  • isatty
  • link
  • unlink
  • mkdir

[ammrat13]

Question: Do we want to support all the system calls listed above for the Virtualfs capability? If not, which ones do we want to support. I guess the question becomes what we want the students to be able to do.