To ensure metadata is always stored, the CID returned from `addFile` will resolve to a directory.

The `path` property is now required of the import candidate passed to `addFile`, if it's not necessary to store a path as part of a file, use `addBytes` or `addByteStream`.

Since `urlSource` returns a `{ content, path }` candidate, it can be used with `addFile`.

To import directly from a URL without a path, a `urlByteStream` function has been added for use with `addByteStream`.

Fixes #707

BREAKING CHANGE: addFile requires the import candidate to have `path` and `content` properties and the returned CID will always resolve to a directory

---------

Co-authored-by: Daniel N <2color@users.noreply.github.com>

Splits the unixfs/mfs stat command return types into two types - "regular" stats (these only return stats available on the root node of the DAG) and "extended" - these collect stats from the DAG, potentially going to the network to fetch missing blocks.

There are separate simple and extended types for files and dirs, but raw/leaf nodes only get one type since there are never any nodes linked to from them.

As a bonus we can now calculate the size of directories correctly, (as the combination of all child files/directories), assuming all blocks are present in the blockstore or fetchable from the network.

Fixes #580

BREAKING CHANGE: Fields that would involve DAG traversal have been removed from the output of `fs.stat` - pass the `extended` option to have them returned

---------

Co-authored-by: Daniel Norman <1992255+2color@users.noreply.github.com>
Co-authored-by: Daniel N <2color@users.noreply.github.com>