Fixes #50.

Add `PartialOrd` and `Ord` implementation to `ArrayString`

What the test was doing was simply UB and we can't expect rustc to make
sense of it. The test was failing with optimizations on, yet a closer look
says arrayvec is still working as it should. So remove the broken test.

Fix 1 << 16 size array impl: only possible on bigger than 16-bit (0.3 branch)

```
warning: the type of this value must be known in this context
   --> src/lib.rs:312:32
    |
312 |                 ptr::copy(p, p.offset(1), len - index);
    |                                ^^^^^^
    |
    = note: #[warn(tyvar_behind_raw_pointer)] on by default
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #46906 <rust-lang/rust#46906>
```

Fix a broken size test in nodrop; this test does not tell us about enum
layout optimizations.