Rust: Match lex return type with other signatures

[marcoroth]

This pull request refactors the lex return type in the public Rust API. Additionally, the LexResult and ParseResult types now implement Display for easier printing.

use herb::{lex, parse};

fn main() {
  let template = "<h1><%= title %></h1>";

  match lex(template) {
    Ok(result) => { println!("{}", result); }
    Err(error) => { eprintln!("Lex error: {}", error); }
  }

  match parse(template) {
    Ok(result) => { println!("{}", result); }
    Err(error) => { eprintln!("Parse error: {}", error); }
  }
}