7708 - baremetal/shell: word data structure

Not everything here is tested, but enough that I'm starting to feel confident.

We see our first divergence with apps/tile. In apps/tile we render everything,
then go back and figure out where to position the cursor. This relies on
some low-level smarts and is also quite klunky and complex.

In baremetal/shell I plan to do something simpler: maintain a tree of objects
where each level knows which sub-object under it has the cursor. Now I
can pass in the cursor object to each object, and if it detects that it
has the cursor it can recursively figure out which sub-object has the cursor.
The bottom-most objects (grapheme stacks) draw the cursor as they render
themselves. Single-pass algorithm, draw the cursor as you render, no low-level
smarts needed.

But there's a divergence. What in apps/tile used to look like this, with
a cursor ␣ at the end of the word 'abc':

  abc␣def

..now looks like this:

  abc␣ def

..with an extra space.

This could cause some jarring 'dancing' as you move the cursor through a
list of words.

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