Expose inner `GcRefCell` as `CollectRefCell` and also add `CollectCell`

[kyren]

These types are generally useful, even if they require unsafe to be useful.

It's nice to not have to manually make an unsafe Collect impl, rather than only having to remember to trigger a write barrier.

[kyren]

If anyone has better names than CollectRefCell and CollectCell I'm all ears.

[moulins]

Instead of having a separate method for each unsafe operation on each cell type, I'd propose just providing a single method:

unsafe fn as_(ref_)cell(&self) -> &(Ref)Cell<T>;

This is slightly more verbose to use, but IMHO, it has two advantages:

• The user can call any method on the Cell/RefCell, not just those we explicitely support;
• Using a specific method name clarifies what "kind" of unsafety is happening.

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If anyone has better names than CollectRefCell and CollectCell I'm all ears.

Some suggestions:

• Lock/RefLock: this is what I'm using in my gc-extra experiment;
• Jail/RefJail: cute variation on Cell;
• FrozenCell/FrozenRefCell: a little verbose, but maybe more explicit?