Borrow - experimental sketch, DO NOT MERGE

[ijackson]

In #22 I wrote

An alternative would be to have a nonoptimised UnicaseNoOpt<str> which could be transparent, so that &'s UnicaseNoOpt<str> can be made from &'s str with one transmute.

This problem nerdsniped me. The result is this branch. Now we can do this:

    let mut hm: HashSet<UniCase<String>> = Default::default();
    hm.insert(UniCase::new("ascii".to_string()));
    assert!(hm.contains(UniCaseNoOpt::from_ref("Ascii")));

which is great! My actual code is not really finished - no docs, probably style problems, and many trait impls etc. missing. (It does pass miri, which is a thing I always check with unsafe)

But I thought I should share it. If people like this direction, it could be tidied up.

[ijackson]

Obviously this direction is not compatible with #36 which forbids unsafe. I think Borrow is rather more important.