librustc: Traverse arbitrarily deep for nullable enum opt.
librustc: Try looking in tuple fields for nullable enum opt.
librustc: Try looking in fixed sized arrays for nullable enum opt.
Add tests.
librustc: Add NonZero lang item and use it if possible for nullable pointer enum opt.
libcore: Add NonZero lang item and implement some methods.
libcollections: Use NonZero in Vec.
liballoc: Use NonZero in Rc.
liballoc: Use NonZero in Arc.
libcore: Make it unsafe to create NonZero and impl Deref.
Add tests for NonZero.
Don't expose NonZero through libstd.
libcore: Don't impl RawPtr* traits for NonZero.
librustc_trans: Get rid of unnecessary allocation in finding discriminant field.
libcore: Use Zeroable trait to try to limit what types may be used with NonZero.
libcoretest: Add tests for NonZero.
libcollections: impl Send/Sync for Vec.
auto merge of #19765 : luqmana/rust/nonzero-lang-item, r=nikomatsakis

This extends the nullable enum opt to traverse beyond just the first level to find possible fields to use as the discriminant. So now, it'll work through structs, tuples, and fixed sized arrays. This also introduces a new lang item, NonZero, that you can use to wrap raw pointers or integral types to indicate to rustc that the underlying value is known to never be 0/NULL. We then use this in Vec, Rc and Arc to have them also benefit from the nullable enum opt.

As per https://github.com/rust-lang/rfcs/pull/499 NonZero is not exposed via the `libstd` facade.

```
x86_64 Linux:
                        T       Option<T> (Before)      Option<T> (After)
----------------------------------------------------------------------------------
Vec<int>                24          32                      24
String                  24          32                      24
Rc<int>                 8           16                      8
Arc<int>                8           16                      8
[Box<int>, ..2]         16          24                      16
(String, uint)          32          40                      32
```

Fixes #19419.
Fixes #13194.
Fixes #9378.
Fixes #7576.