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.

1 files changed, 2 insertions, 2 deletions

diff --git a/examples/toggle.aup b/examples/toggle.aup
index 39417cc..d5cf5d9 100644
--- a/examples/toggle.aup
+++ b/examples/toggle.aup
@@ -1,10 +1,10 @@
 # Space toggles visibility; Right arrow moves a position that wraps.
-state visible = 1
+state visible = true
 state position = 120
 input toggle = key Space
 input move = key ArrowRight
 view marker_x = position
-view marker_radius = visible * 24
+view marker_radius = if visible { 24 } else { 0 }
 view tile_x = position - 24
 view value = visible