Add operations for mutate write::DebuggingInformationEntry

[yurydelendik]

Are there operations for remove the child DIE or attributes? It will be useful to have such operations to mutate in-memory UnitTable forest.

[philipc]

Not yet, but we definitely want them.

For DIEs, removing them just needs to update the children field of the parent, without actually deleting the child from the entries Vec. Attributes can simply be deleted.

One catch with that will be if a deleted attribute referenced something in another table (strings or range lists), then we'll still write those values out (deleting them from the other tables isn't easy). I don't have a solution for that yet.