(PUP-2738) FFI from_string_to_wide_string block

 - MemoryPointer from_string_to_wide_string previously allocated a
   new MemoryPointer instance but didn't force callers to scope / free
   it.  This allowed the Ruby runtime to clean it up on GC, but we
   prefer to force callers to use a block form, to clean up the
   unmanaged memory when the variable is no longer needed.  Refactor
   the method to yield the ptr.
 - Since the ptr is no longer valid after the block yields, return nil
   from the method
 - Refactor valid_sid? to read ptr inside block, use closure to store
   the 'valid' return value.
(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. Structure code to ensure that
   a REPARSE_DATA_BUFFER is returned from resolve_symlink rather than
   a FFI::MemoryPointer
 - Fix bug in Puppet::Util::Windows::File.resolve_symlink that could
   result in the wrong value being returned.
(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 we capture the
   exit_status within the block, and return it outside of the block
 - Fix bug in Puppet::Util::Windows::Process.process_privilege_symlink?
   that could result in the wrong value being returned.
 - Fix bug in Puppet::Util::Windows::Process.elevated_security?
   that could result in the wrong value being returned.
(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.
 - Ensure that we don't use non-local return's anywhere, due to their
   potential dangerous / unexpected behavior.  When inside blocks,
   store off return values in a separate variable to be returned
   outside the block.  When values are only used inside yields, and the
   corresponding unmanaged memory has been freed, return nil outside
   the block.