FastText struct shouldn't implement clone

[caio]

Hello,

First of all, thanks a lot for your work on this crate!

Cloning doesn't play well with the custom drop logic, it's better to remove the Clone bit from derive and let users share it via Arc/Rc I think, otherwise this can easily happen:

use fasttext::FastText;

fn main() {
    let mut ft = FastText::new();
    ft.load_model("cooking.model.bin").unwrap();

    let cloned = ft.clone();
    drop(cloned);

    // boom!
    ft.is_quant();
}

Running it causes a 'cargo run' terminated by signal SIGSEGV (Address boundary error)

[messense]

Thanks for reporting.