Incorrectly uses usize instead of u64 for file offsets

[peter-bertok]

As per the discussion here: https://users.rust-lang.org/t/rust-beginner-notes-questions/14928/81?u=peter_bertok

The file offset should be consistently u64 everywhere instead of usize, even on 32-bit platforms. In practice, I believe Windows doesn't actually support mapping more than 2 GB on 32-bit, so isize::MAX might be a better window-size limit.

[dekellum]

Might this be fixed by #65?

[Dr-Emann]

I believe this was indeed fixed by #65, merged in #71