Add an option to verify Map() writes don't overrun. Closes #42

* Has a couple of limitations - won't check deferred context or
  NO_OVERWRITE Map()s except in a captured frame. This could in theory be
  implemented but it'd be complex and I don't want to complicate/break
  the normal path.
* When an overrun is detected, a messagebox pops up to block the thread,
  and if you hit yes, it will debugbreak.