lazy/lazy.bend checks

lazy: branches that do not run both ways. Bool.pick, Bool.and and Bool.or are ordinary defs, so every argument is evaluated before the call: a search written as Bool.pick(T, hit, here, go(rest)) walks the whole list whatever hit says (measured native, 100 iterations over 100k cells: the hit at the head costs 0.10 s, the same as the hit at the end, while the forms here cost 0.00 s). A thunk Unit -> T is a value, so the branch that is not taken is never applied.