Add '#' and '.' special registers

These come from Kakoune:

* '#' is the selection index register. It's read-only and produces the
  selection index numbers, 1-indexed.
* '.' is the selection contents register. It is also read-only and
  mirrors the contents of the current selections when read.

We switch the iterators returned from Selection's `fragments` and
`slices` methods to ExactSizeIterators because:

* The selection contents register can simply return the fragments
  iterator.
* ExactSizeIterator is already implemented for iterators over Vecs, so
  it's essentially free.
* The `len` method can be useful on its own.

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