[AST] NFC: Refactor ASTDumper to use a "label" abstraction.

ASTDumper allows nodes/values to be printed without labels, which
works fine for the default output but won't work for JSON when every
value needs to have a valid key. To balance these needs, we create
a `Label` abstraction that can be created as either `always` or
`optional`. All the current labels are treated as `always`, and all
other values have had `optional` labels associated with them, which
won't be printed in the default AST dump.

This change also adds a `printList` function that replaces anywhere
that `for` loops with `printRec` calls in their bodies. This will
be used to provide the necessary array structuring for JSON output
later. (There are some places where `for` loops call `printFlag`
which will need to be dealt with later.)

Author: allevato