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

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`](https://docs.rs/curl/0.4.47/curl/easy/struct.Easy2.html#method.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