Extend and improve FcPattern bindings

The fontconfig `FcPattern` type is wrapped as `fc::Pattern` and
`fc::Pattern` ref. All methods for accessing data on the pattern now
return an `Iterator`. This API turns out to be much more ergonomic than
providing an integer index. We also override the default `nth`
implementation of `Iterator` on these accessors to allow random
(incremental only) access.

For instance, accessing `family` attributes from a pattern:

    let families = pattern.family();
    let second = pattern.nth(1);

Or printing available styles

    for style in pattern.style() {
        println!("style={}", style);
    }

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