Clarify the decision behind the HWND/HANDLE/Ptr() type synonyms

I was surprised to find that these aren't the definitions for `HWND` and `HANDLE`

`newtype HWND = HWND { unsafeHWNDToHANDLE :: HANDLE }`

`newtype HANDLE = HANDLE { unsafeHANDLEToPtr :: Ptr () }`

I'm not an expert in Win32 programming, but I don't think that conversions between these three types are so pervasive (and without consequence) as to necessitate using synonyms here rather than something which protects the programmer a little more. If this is required, it would be great to document the declarations with this information, if not then perhaps there could be a major version update replacing the synonyms.

The same thing could probably be said for any of the other type synonyms of `HANDLE` in `Win32`.

Begin with the HWND, HANDLE, and other HANDLE-related type-synonym declarations in Win32, then review how their conversions are used. Done means documenting the rationale for keeping the synonyms, or reaching a project decision about replacing them with newtypes in a major version.

Written by the indexing model from the issue text.