Implements Iterator for Leds

[rubberduck203]

• Deprecates into_array
• Appears to save quite a bit of code size over the old method of calling into_array().iter_mut() by implementing Iterator for 'a &mut Leds directly.

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James Munn helpfully pointed out that this saved an actual ~500 bytes, which is far more in line with what I expected to save.
Still a huge win.

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A more controlled experiment suggests this saves 796 bytes over calling leds.into_array().iter_mut().

27c27
<     let leds = Leds::new(
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>     let mut leds = Leds::new(
40,41d39
<     let mut arr = leds.into_array();
< 
45c43
<         for led in arr.iter_mut() {
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>         for led in &mut leds {