- change offset to u64
- make len non-optional
- make write take a file_size parameter
- std.Io.Threaded: introduce disable_memory_mapping flag to force it to
  take the fallback path.

Additionally:

- introduce BlockSize to File.Stat. On Windows, based on cached call to
  NtQuerySystemInformation. On unsupported OS's, set to 1.
- support File.NLink on Windows. this was available the whole time, we
  just didn't see the field at first.
- remove EBADF / INVALID_HANDLE from reading/writing file error sets