Automatically shard directories

https://github.com/ipld/js-unixfs/pull/41 added HAMT sharded directories support but it is opt in - you need to explicitly use the sharded directory writer.

In most IPFS implementations, the use of sharded directories (or not) depends on the byte size of the directory block with all the items in it. i.e. if the directory block exceeds a given size it _automatically_ becomes a sharded directory.

Start by reading the linked pull request #41 and the sharded directory writer it introduced. Trace how directory block size is determined and where callers choose the writer; done means that choice is automatic when a directory block exceeds the agreed size, without requiring callers to opt in.

Written by the indexing model from the issue text.