(PUP-2382) Create FFI typedefs for Win32 API

 - Add new file windows/api_types to contain global FFI typedefs
   that can be reused wherever we attach_function for FFI
 - Replace existing function definitions with new Win32 API style
   typedefs such as :dword, :lpcstr, :handle, etc
 - Fix some API definitions that were using incorrect FFI typedefs
   such as :ulong and :long.  In FFI, these typedefs are platform
   specific and will vary in size.  On Windows, these are fixed width.
   New Win32 typedefs of :win32_ulong and :win32_long were added
   to accommodate these in function definitions.
 - Fix replace_file by passing FFI::Pointer::NULL to match it's updated
   definition.
 - Where applicable, modify FFI::MemoryPointer.new to use the Win32
   typedef being created, so .new(:dword, 1) instead of .new(:uint, 1)
 - Prefix each attach_function with an ffi_lib for the dll it's
   actually loaded in.
 - Add missing documentation / links where helpful.

Paired-with: Rob Reynolds <rob@puppetlabs.com>
(PUP-2382) Standardize FFI module conventions

 - Extend FFI with attach_function_private, which allows adding an FFI
   definition to a module privately.  This is a bit simpler than nesting
   an 'API' module / clearer than FFI definitions inside a private.
 - Unfortunately this doesn't make class / struct definitions private
   but this shouldn't be an issue in a practical sense (and isn't a
   change to the previous behavior / design)
 - Refactored existing API definitions and settled on mirroring the
   Win32 names precisely.  In some modules, the API names had been
   ruby-ified, which led to collisions.  Using the casing of the Win32
   API calls is a sufficient hint to differentiate them from Ruby code.
 - Found missing values for TOKEN_INFORMATION_CLASS that were added to
   enum
(PUP-2382) FFI Puppet feature symlink test
(PUP-2382) Require windows/string earlier

 - To allow using windows/string as a mixin, require it earlier