NTAs must have a declared `NTAOwner` parent that enumerates them,
which should cover most use cases.

Since SmallSet is not API-compatible to java.util.Set, SmallSet
attributes must be declared as SmallSet<T>, not Set<T>.

New command line options:
-e (--attr-extract) can extract attribute values; combine with
   '-o' (or stdout redirection) to write to a file.
-d (--attr-diff) can compare against a file previously extracted via
   -e.  Prints a diff, and the exit code indicates whether there was a
   difference (as with `diff`).

The output files are designed to be easy to process via awk and
similar line-by-line tools.

Internally, the differentiator is set up to (potentially) be able to
also report to CodeProber without going through stringification, and
to ignore selected nodes during differencing.

Also ensure that we report syntax errors.  Currently piggy-backing on
`Program.reports()' to avoid creating more automatic probes that
pollute the UI.

Disabling individual UI elements is not supported by the current
code-prober yet, so this is part is currently non-functional.
Students need to be careful to not accidentally toggle options that
can lead to subtle changes in behaviour, so we should ideally fix that
before the course starts-- however, the "forced defaults" offer a
workaround, by essentially overriding attempts to change configuration
elements.