Add count, sum, max and min over a list

count(t in tasks where !t.done), sum(t.points for t in tasks), and max
and min in the same form compile to Pure's FOLD: the value and the where
condition become a block run once per item, as a column of the list, so
views count and add without a copy of the list or a state kept in step.
max(a, b), min(a, b) and fns of the same names stay calls, told apart by
the `for` after the first expression or the `name in` after count(.

Inside the block only its item and states can be read; a parameter of
the fn outside, the enclosing column's row, dt and a fold inside a fold
are compile errors that say why. The checks run 3,600 random steps
against a Python model and add seven diagnostics.

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