Rewrite printing of #[...] forms, for simplicity now and later for sharing.

Rewrite printing of #[...] forms, for simplicity now and later for sharing.

In order to properly implement datum labels for things inside of bracketed
forms, we need to be able to walk the items that are being printed there.
Previously this would have been very difficult since the insides of these forms
were printed by custom printers.  Now they are exposed as individual objects
that can be walked.

Further work is necessary to include these objects when walking the tree.