Stalls and consumes all RAM when given the `ffi` ligature

[rspeer]

If I run default_case_fold_str on a string containing the ligature ffi, it appears to cause an infinite loop that consumes all RAM.

Here's a test case that encounters the infinite loop:

    #[test]
    fn test_ligature() {
        let s = "the spiffiest test";
        assert_eq!(default_case_fold_str(&s), "the spiffiest test");
    }

(In case font rendering is obscuring the problem, the input word "spiffiest" is made of 7 characters: ['s', 'p', 'i', 'ffi', 'e', 's', 't'].)

[AndrewKvalheim]

Resolved in 27ceca4?