Fix parsing and evaluation of 'all' and 'show'.  See comment in evaluatingParser.
Fix closeable streams to recreate streams for a key if the delegate was closed.
Include the key name in the streams backing file path to avoid collisions.
TaskKey[T].previous: Option[T], which returns the value of the task the last time it executed.

This requires a Format[T] to be implicitly available at the call site and requires the task
to be referenced statically (not in a settingDyn call).  References to previous task values
in the form of a ScopedKey[Task[T]] + Format[T] are collected at setting load time in the
'references' setting.  These are used to know which tasks should be persisted (the ScopedKey)
and how to persist them (the Format).

When checking/delegating previous references, rules are slightly different.

A normal reference from a task t in scope s cannot refer to t in s unless
there is an earlier definition of t in s.  However, a previous reference
does not have this restriction.  This commit modifies validateReferenced
to allow this.

TODO: user documentation
TODO: stable selection of the Format when there are multiple .previous calls on the same task
TODO: make it usable in InputTasks, specifically Parsers