feat: add config option to control fanout size (#356)

* feat: add config option to control fanout size

Adds a `shardFanoutBytes` option to the importer to allow configuring
the number of bytes used for the HAMT prefix, also a test.

* fix: use fanout "bits" (#357)

---------

Co-authored-by: Rod Vagg <[email protected]>

Author: achingbrain

packages/ipfs-unixfs-exporter/src/resolvers/unixfs-v1/content/hamt-sharded-directory.ts

@@ -33,7 +33,7 @@ async function * listDirectory (node: PBNode, path: string, resolve: Resolve, de
     throw errCode(err, 'ERR_NOT_UNIXFS')
   }
 
-  if (!dir.fanout) {
+  if (dir.fanout == null) {
     throw errCode(new Error('missing fanout'), 'ERR_NOT_UNIXFS')
   }