Let's support Display

[pavloslav]

It's faster and more rust-way to write! everything into Formatter than creating a Vec<String>, .joining them and returning. Those who need a string still can use to_string method instead of to_words.

[pavloslav]

I've played a bit with this idea.

I think we can do that, but right now I don't clearly see the interface.

Display::fmt returns std::fmt::Error only, so Num2Err should be returned before that. Something like

println!("I'll pay you {} for a bottle of beer", Num2Words::new(15.20).prefer("USD").build()?); //we need an additional .build()? here

Currently, Rust doesn't support custom format specifiers. There is custom_format crate, but I don't think it's a perfectly good idea to add it as dependency.

We can do something like the last comment here; but I don't like the idea to boilerplate 5 sametypes for each language (but maybe we can do it with a macro?).

Any better ideas?