Add Font::codepoint_ids

[alexheretic]

Provide an abstraction for mapping GlyphId -> char.

pub trait Font {
    fn codepoint_ids(&self) -> crate::CodepointIdIter<'_>;
}

let font = FontRef::try_from_slice(include_bytes!("../../dev/fonts/Exo2-Light.otf"))?;

// Iterate over pairs, each id will appear at most once.
let mut codepoint_ids = font.codepoint_ids();
assert_eq!(codepoint_ids.next(), Some((GlyphId(408), '\r')));
assert_eq!(codepoint_ids.next(), Some((GlyphId(1), ' ')));
assert_eq!(codepoint_ids.next(), Some((GlyphId(75), '!')));

// Build a lookup map for all ids
let map: HashMap<_, _> = font.codepoint_ids().collect();
assert_eq!(map.get(&GlyphId(75)), Some(&'!'));

Resolves #18

[robinkrahl]

Looks good to me. Thanks for implementing this so quickly!