Add a Share kind

Fixes #11781
Make `Rc`, `Cell` and `RefCell` NoShare
Replace Freeze bounds with Share bounds
Add Unsafe<T> type
Make Cell and RefCell use Unsafe<T>
Let ArcData use Unsafe<T>
Gather loans for static items

We currently gather loans for static items that are defined within
functions. This change enables loan gathering on static items declared
globally.
Forbid borrow of static items with unsafe interior
Allow static items that don't fulfill `Freeze`
Make atomics interior Unsafe<T>
Mention share in guid-unsafe instead of freeze
Mention Share in the tutorial
auto merge of #12686 : FlaPer87/rust/shared, r=nikomatsakis

`Share` implies that all *reachable* content is *threadsafe*.

Threadsafe is defined as "exposing no operation that permits a data race if multiple threads have access to a &T pointer simultaneously". (NB: the type system should guarantee that if you have access to memory via a &T pointer, the only other way to gain access to that memory is through another &T pointer)...

Fixes #11781
cc #12577 

What this PR will do
================

- [x] Add Share kind and
- [x]  Replace usages of Freeze with Share in bounds.
- [x] Add Unsafe<T> #12577
- [x] Forbid taking the address of a immutable static item with `Unsafe<T>` interior

What's left to do in a separate PR (after the snapshot)?
===========================================

- Remove `Freeze` completely