This introduces a `Token` in place of the original tuple for
`TreePreOrderDfs` so that it can be used as input to a parser that will
lower into XIRF.
This requires that various things be describable (using `Display`), which
this also adds. This is an example of where the parsing framework itself
enforces system observability by ensuring that every part of the system can
describe its state.
DEV-13708
The `TreePreOrderDfs` iterator needed to expose additional edge context to
the caller (specifically, the `Span`). This was getting a bit messy, so
this consolodates everything into a new `DynObjectRel`, which also
emphasizes that it is in need of narrowing.
Packing everything up like that also allows us to return more information to
the caller without complicating the API, since the caller does not need to
be concerned with all of those values individually.
Depth is kept separate, since that is a property of the traversal and is not
stored on the graph. (Rather, it _is_ a property of the graph, but it's not
calculated until traversal. But, depth will also vary for a given node
because of cross edges, and so we cannot store any concrete depth on the
graph for a given node. Not even a canonical one, because once we start
doing inlining and common subexpression elimination, there will be shared
edges that are _not_ cross edges (the node is conceptually part of _both_
trees). Okay, enough of this rambling parenthetical.)
DEV-13708
Mike Gerwitz