(PUP-2881) Refactor ITaskTrigger for FFI / COM

 - Define ITaskTrigger COM interface and the relevant struct types using
   FFI
 - Update low-level memory copying and function call definitions to use
   COM wrappers
 - Properly allocate / cleanup memory with FFI pointer block form
 - No need to manually check HRESULT failure, as the COM wrappers
   will automatically throw exceptions when FAILED HRESULTs encountered
 - Refactored redundant code that tasks a Ruby hash and populates the
   ITaskTrigger object into the populate_trigger method - this is called
   by new_work_item, trigger= and add_trigger
 - Refactor COM struct to Ruby hash conversion code into
   populate_hash_from_trigger
 - Had to extend our TaskScheduler::COM with FFI::Library to gain
   enum definition abilities
(PUP-2881) Remove msvcrt/buffer from taskscheduler

 - Last appendage hanging on from windows-pr
(PUP-2881) Remove taskscheduler S_OK constant