ipld/unixfs/io: better initialization and options for Directories

WithMaxWidth()
--------------

The new option sets effectively a maximum width to the
directories.

Currently, on a dynamic directory, the switching to-from Basic to HAMT is
controlled by HAMTShardingSize, and the width of the HAMT shards by DefaultShardWidth.

When WithMaxLinks() is specified, the switching is additionally controlled by
the number of links exceeding the limit. In that case, MaxLinks is used as ShardWidth.

The directory can only be converted back to BasicDirectory when the total
number of links is below the limit.

Backwards compatibility is kept and tests have been added.

Note that when setting MaxLinks to a high number, it is possible that we still
suffer automatic conversion to HAMT before hitting MaxLinks, if the estimated
directory size is above 256KiB (as before).

WithStat()
----------

Allows to set Stat on a new directory.

WithCidBuilder()
---------------

Allows to set the CidBuilder on a new directory.

Author: hsanjuan

CHANGELOG.md

@@ -16,6 +16,12 @@ The following emojis are used to highlight certain changes:
 
 ### Added
 
+- `ipld/unixfs/io/directory`: We have made some changes to unixfs directory tooling:
+  - We have exposed creator methods for new `BasicDirectory` and `HAMTDirectory`, that complement the existing `NewDirectory()` which creates dynamic directories.
+  - We have added `WithCidBuilder(...)` and `WithMaxLinks(...)` as options to these new methods so that empty directories can be initilized as wished from the get-go.
+  - `WithMaxLinks(...)` is a new option that allows to set a limit to the number of children that a directory DAG node can have. For details on what exactly it does for each of the directory types, please check the documentation.
+  - `WithStats(...)` is a new option that allows to set the Stat information for the new directory node.
+
 ### Changed
 
 - upgrade to `go-libp2p` [v0.41.1](https://github.com/libp2p/go-libp2p/releases/tag/v0.41.1)