Incorrectly uses usize instead of u64 for file offsets #67
Description
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.