RFC: Introduce Mut<T>, remove Cell<T>

`Mut<T>` implements an internally mutable slot, using `#[no_freeze]` and `transmute_mut`. It is much like Cell without the nullable part. It implements dynamic borrow checking, which is just an enum counting the current readers and writers.

`Mut<T>` provides methods `.borrow() -> ReadPtr<'a, T>` and `.borrow_mut() -> WritePtr<'a, T>` to obtain a wrapped borrowed pointer. These wrapped pointers have destructors that update the borrow flag in the `Mut<T>` when they go out of scope. Several `ReadPtr` can be alive at the same time.

Example:

    let obj = Mut::new(5);
    {
        let mut m_ptr = obj.borrow_mut();
        *m_ptr.get() = 7;
    }
    do obj.map |t| {  println!("value: {}", *t) }

For convenience, and to remove Cell, extra methods are implemented on `Mut<Option<U>>` that make it easy to treat it like an optional value. When replacing Cell's `.take()`, the only overhead of `Mut<T>` at the moment is the destructor function call of `util::NonCopyable` that's inserted.

`Mut<T>` is more generic than `Cell<T>`, and so can replace it better when it is used as an actual mutable slot. This represents very few previous users of Cell in the tree (see commit https://github.com/blake2-ppc/rust/commit/4f8c5b1954aef4e01c40935708757373d9b04b6f )

`Mut<T>` is also used to replace `extra::rc::RcMut<T>` with `Rc<Mut<T>>`. This requires a further restriction for cycle prevention (unless I'm mistaken) so that only `T: Send` values are allowed inside `Rc<Mut<T>>`. (This is because `Rc<Mut<T>>` *is* `Freeze`, while `RcMut<T>` was not.)

### Clarifications and Known issues:

* It doesn't merge. I'll wait for #9359 before rebasing, and I'll update the commit messages to be more detailed.
* The `BorrowFlag` enum is 16 bytes and is too large. This can be solved inelegantly by simply using an integer.
* `Mut<T>` should be noncopyable because it needs to clear the borrow status when cloned.
* For pcwalton: the name `Mutable<T>` was not possible since it conflicts with `std::container::Mutable`

### Requests for comment:

* Cell<T> is replaced with a more fundamental type while 90% of users don't need this. Should Cell stay with just the constructor and the single method `.take()`?  This represents the use case where closures wish to modify their moved-in upvalues.
* Is it acceptable for a type to have both `impl<T> Mut<T>` and `impl<U> Mut<Option<U>>` ?
* Should `Mut<T>` allow multiple `ReadPtr` (and thus it has to count them)?