Give aup a bool: true and false, and conditions that take one

A condition was any number, non-zero being true: done was a number field,
a view showed 1 for true, and done + 1 was no type error. Now bool is its
own type (Pure 016, module format v6):

- true and false; comparisons, text comparisons, has and ! && || give a
  bool. Bools are equal or not; they are no numbers and have no order.
- if, conditional expressions, require, where (folds, walks, filtered
  tables) take a bool. Given a number, the diagnostic says how to make
  one: "if takes a bool, not a number: compare it, as in n != 0".
- Bool states (state on = false), record fields (done = false), message,
  output and argument fields (ok: bool), lets, views and columns.
  Interpolation writes true or false. A list is not keyed by a bool.
- The examples' flags are bools (charge's holding, toggle's visible,
  tasks' done).

Checks: 20 bool diagnostics, a form using bools everywhere, an app whose
every output shows true and false (states, items, views, columns, text)
and whose replacement turns a number into a bool and back. Models hold
bools and compare them strictly: Python's True == 1 no longer hides a 1
where a true belongs. Generated programs are typed, numbers and bools.

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