Remove SpannableList

[schultzcole]

INodeDataSource.CopyNodeBytesTo should be replaced with something like INodeDataSource.VisitNodeBytes(ulong hash, Action<Span<byte>>

SpannableList should probably just be replaced by a List, and use CollectionsMarshal to get a span in VisitNodeBytes. The list would have to be guarded by a lock in this case, but I think this is acceptable.

[schultzcole]

Rather than using a normal list, it might be safer to introduce AppendOnlyList, which would keep a list of arrays, adding a new array when the capacity is exceeded.

This would mean that you can give out a span referencing the internal arrays without worrying that it might become stale, since the internal arrays is not re-allocated. And since writes are append only, you don't need to lock for reads.

Caveats:

• Storage wont be completely contiguous.
• Because storage won't be contiguous, the data for any given node can't bridge between dis-contiguous backing arrays.
  • This means that if we're writing 6 bytes and there's only 5 bytes of capacity, we need to allocate a new array, leaving those 5 bytes unused.
  • This is probably not a huge issue unless a really giant node is added that is just above the remaining capacity of the list.
  • It might be possible to mitigate the likelihood of this happening by being smart about how we store large nodes. For instance, it might be worth storing collections as a linked list of nodes, each of which contains a subset of the list bytes (maybe 32 or so, IDK).

It may be worth introducing a "defragment" operation for the list that can run to consolidate the sub-arrays into a single array, reducing the number of unused bytes. This would, however, introduce the need for a locking mechanism while the defragment is being performed.

We should benchmark to see whether AppendOnlyList or List with a lock is faster for reading/writing