Forward-reverse grapheme mismatch on "\u{1F938}\u{1F3FE}\u{1F3FE}"

[Manishearth]

Found by cargo-fuzz.

This crashes:

extern crate unicode_segmentation;

use unicode_segmentation::UnicodeSegmentation;
fn main() {
    let s = "\u{1F938}\u{1F3FE}\u{1F3FE}";
    let forward = UnicodeSegmentation::graphemes(s, true).collect::<Vec<_>>();
    let forward_reversed = forward.into_iter().rev().collect::<Vec<_>>();
    let reverse = UnicodeSegmentation::graphemes(s, true).rev().collect::<Vec<_>>();
    assert_eq!(forward_reversed, reverse);
}

It panics with (manually escaped):

left `["\u{1F938}\u{1F3FE}\u{1F3FE}"]`, right: `["\u{1F3FE}", "\u{1F938}\u{1F3FE}"]`

The original emoji is (uniview) U+1F938 PERSON DOING CARTWHEEL followed by two Fitzpatrick skin color modifiers. I suspect this error will happen whenever you have two skin color modifiers after a modifiable emoji.

cc @mbrubeck @kwantam

[Manishearth]

We have another mismatch on \u{200D}\u{200D}\u{2764}\u{2764}; the forward iterator considers it a single grapheme but the reverse iterator splits off the last heart into its own grapheme. This is likely a separate bug, but I'm listing it here.

[HadrienG2]

Should be fixed by #23 .