The "GlyphIter" function should use two seperate lifetimes for its iterator.

GlyphIter is currently defined as:

pub struct GlyphIter<'b, I: Iterator>
where
    I::Item: IntoGlyphId,
{
    font: &'b Font<'b>,
    itr: I,
}

However, consider the case where 'b is static (e.g. I have loaded the Font class using the try_from_vec function). When glyphs_for is called on the Font, it finds that the anonymous reference used for Font is not 'static, so Font is downgraded to some kind of non-static font. This has caused problems for me in my code. The easiest fix for this would be to have two lifetimes in the GlyphIter: one for the borrowed Font and one for the lifetime in the Font object. I am willing to implement a pull request for this issue.