utf-8 comparisons index out of range

[experimenti]

Thanks for the contribution. The performance is great.

Getting an error on text with extended encoding:

The runtime error:
<unnamed>' panicked at 'index out of bounds: the len is 17 but the index is 17', /rustc/6afcb5628523b0baae5704ad34ac1aba8ba10de6/src/libcore/slice/mod.rs:2695:10

1 failing unit test

 #[test]
    fn encoded_case() {
        assert_eq!(1, simple("LA CONNEXION FRANÇAISE LTD", "LA CONNEXION FRANÇAISE LTD"));
    }

Trace

   7: core::panicking::panic_fmt
             at src/libcore/panicking.rs:85
   8: core::panicking::panic_bounds_check
             at src/libcore/panicking.rs:61
   9: <usize as core::slice::SliceIndex<[T]>>::index
             at /rustc/6afcb5628523b0baae5704ad34ac1aba8ba10de6/src/libcore/slice/mod.rs:2695
  10: core::slice::<impl core::ops::index::Index<I> for [T]>::index
             at /rustc/6afcb5628523b0baae5704ad34ac1aba8ba10de6/src/libcore/slice/mod.rs:2552
  11: <alloc::vec::Vec<T> as core::ops::index::Index<I>>::index
             at /rustc/6afcb5628523b0baae5704ad34ac1aba8ba10de6/src/liballoc/vec.rs:1687
  12: sift4::sift4_offset
             at src/lib.rs:64
  13: sift4::simple
             at src/lib.rs:20
  14: sift4::tests::encoded_case
             at src/lib.rs:118
  15: sift4::tests::encoded_case::{{closure}}
             at src/lib.rs:117
  16: core::ops::function::FnOnce::call_once
             at /rustc/6afcb5628523b0baae5704ad34ac1aba8ba10de6/src/libcore/ops/function.rs:231
  17: <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once
             at /rustc/6afcb5628523b0baae5704ad34ac1aba8ba10de6/src/liballoc/boxed.rs:704
  18: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:85
  19: test::run_test::run_test_inner::{{closure}}
             at /rustc/6afcb5628523b0baae5704ad34ac1aba8ba10de6/src/libstd/panicking.rs:272
             at /rustc/6afcb5628523b0baae5704ad34ac1aba8ba10de6/src/libstd/panic.rs:388
             at src/libtest/lib.rs:1467