Add conditional expressions and table views

if c { a } else { b }, with else-if chains, is an expression anywhere a
value goes. Only the chosen branch runs, every branch gives the same type,
and nesting that would pass Pure's limit of 8 value-carrying joins is a
compile error.

view name = each row in list { column = value ... } declares a table with
a row per item; columns read their row as an index of the list. An index
now reads and writes its item's fields directly (at.title, at.done = 1),
so 'position' cannot name a record field.

examples/tasks.aup gains conditional views and a board table and still
matches its reference model; a new check confirms the unchosen branch is
never computed.

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