`Easy2::post_field_size` takes `size` as `u64` instead of `usize`

[svenkata9]

Hi Alex,

Thanks for all your great work on curl-rust. Extremely useful to use Easy/Easy2 safe wrappers.

I just noticed that Easy2::post_field_size takes size as u64 instead of usize. Typically Rust's .len() functions output usize, and typically we send a &str or String whose length we have to convert to as u64 to pass to this function in curl-rust.

Is there an issue in taking in usize?
Thanks,

~Sankar