(PUP-2738) open_symlink should wide_string once

 - open_symlink is the only caller for create_file, and create_file
   already calls wide_string on the first parameter.  Remove
   unnecessary wide_string call perfomed in open_symlink
(PUP-2738) Puppet::Util::Windows::File FFI cleanup

 - Break resolve_symlink into an additional helper to simplify memory
   usage / cleanup.  Each time an FFI::MemoryPointer is encountered,
   it's cleaned up immediately after use.  Note that it was critical to
   add a return inside the resolve_symlink block.  Without the return,
   the value returned from resolve_symlink is the FFI::MemoryPointer
   that references the REPARSE_DATA_BUFFER
(PUP-2738) Puppet::Util::Windows::Process FFI clean

 - Refactor wait_process so that the FFI::MemoryPointer for the exit
   code is cleaned up immediately after use.  Note that it was critical
   to add a return inside the block.  Without the return, the value
   returned creates 'Invalid Memory Object' errors
(PUP-2738) Use block form of FFI::MemoryPointer

 - Existing uses of FFI::MemoryPointer rely on Ruby GC to perform
   non-deterministic memory cleanup.  In an effort to reduce memory
   consumption, use the block form of FFI::MemoryPointer to call free
   on pointers as soon as they're no longer needed.