Move a `Node`'s parent into the descendents list.

`Node` has an optional parent and a list of other descendents. Most of
the time the parent is treated the same as the other descendents --
error-handling is the exception -- and chaining them together for
iteration has a non-trivial cost.

This commit changes the representation. There is now a single list of
descendants, and a boolean flag that indicates if there is a parent (in
which case it is first descendent). This representation encodes the same
information, in a way that is less idiomatic but cheaper to iterate over
for the common case where the parent doesn't need special treatment.

As a result, some benchmark workloads are up to 2% faster.

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