std::str: Use CharIterator in NormalizationIterator

Just to simplify and not have the iteration logic repeated in multiple places.
std::str: Fix bug in .slice_chars()

`s.slice_chars(a, b)` did not allow the case where `a == s.len()`, this
is a bug I introduced last time I touched the method; add a test for
this case.
extra::dlist: Fix bug in Eq::ne
auto merge of #8858 : blake2-ppc/rust/small-bugs, r=alexcrichton

Fix a bug in `s.slice_chars(a, b)` that did not accept `a == s.len()`.

Fix a bug in `!=` defined for DList.

Also simplify NormalizationIterator to use the CharIterator directly instead of mimicing the iteration itself.