Bugfix

Bug:
  Snake can still be tricked into turning inside itself (e.g. you're
  going up, and can change to moving down)
How to replicate:
  Using the commit immediately before this, this can easily be
  replicated by going upward, then hitting down and right simultaneously
Fix:
  I added a check for each direction that the piece immediately after
  the head (pieces[1]) is not directly in the direction it's trying to
  go.

  It should be impossible to hit pieces[1]. The minimum piece that's
  "hittable" is pieces[3]