3.0.0

• 3.0 is based on 1.0 and changed:

pub struct Parser<'a, I, O> {
    method: Box<Fn(&mut Input<I>) -> Result<O> + 'a>,
}

to

pub struct Parser<'a, I, O> {
    method: Box<Fn(&'a [I], usize) -> Result<(O, usize)> + 'a>,
}

This is like 2.0 version, but avoids potential issue such as #23.

• Toolchain switched to Rust 2018 stable channel.