On windows, `HANDLE` type values are converted to `syncio.FileHandle` in
`lib/std/syncio.nim`, `lib/pure/memfiles.nim` and `lib/pure/osproc.nim`.
`HANDLE` type is `void *` on Windows and its size is larger then `cint`.
https://learn.microsoft.com/en-us/windows/win32/winprog/windows-data-types
This PR change `syncio.FileHandle` type so that converting `HANDLE` type
to `syncio.FileHandle` doesn't lose bits.
We can keep `FileHandle` unchanged and change some of parameter/return
type from `FileHandle` to an type same size to `HANDLE`, but it is
breaking change.