Clean up node references

[g2p]

First order of business is to remove insertable from the public interface.

add_child should then be reimplemented in terms of fast_insert + InsChild (fast_insert can do more checks, and using the same interface would allow spilling through fast_insert).

Then we get into the question of children vs child_alloc_id semantics. Currently these are made complex due to path dependence. We could simplify these by storing a sum type which is either a loc or an alloc_id, similar to how it was done on raw_node, but with an explicit discriminant.
flush_children will stay separate, because it is (most likely) a much smaller tree.