Trim down the Extending Safe Mutability proposal.

There were holes and distractions in the previous iteration, however perfection is unnecessary so let's just aim for a step forward instead. Besides, the language will continue to evolve, and it will be easier to add it than to remove from it.

Notable changes:
- removed the distraction on layouts, Rust is nominally typed so let us focus on nominal typing.
- removed the vague "do not leak references", it is much too easy to accidentally introduce holes here.
- removed the example of RefCell, and focus on POD and Cell for now.
- reorganized the sections a bit, with more examples.
- added some more unresolved questions to ponder.