GitLab

Add a UnixFS `Directory` that hides implementation details and helps to
distinguish *what* is a UnixFS directory.

Replace the `unixfs.io.Directory` structure that contained the HAMT and basic
directory implementations (through inner pointers) with an interface containing
the same methods. Implement those methods in two clearly distinct structures for
each implementation (`BasicDirectory` and `HAMTDirectory`) avoiding pointer
logic and clearly differentiating which implementation does what.

The potential basic to HAMT transition was being hidden behind the `AddChild`
call at the UnixFS layer (changing one implementation pointer  with the other
one), it is now being explicitly done at the MFS layer.

Rename the `dirbuilder.go` file to `directory.go` and change the `Directory` MFS
attribute `dirbuilder` to `unixfsDir` to be consistent.

License: MIT
Signed-off-by: Lucas Molas <schomatis@gmail.com>

Also:

* Update the blockstore/blockservice methods to match.
* Construct a new temporary offline dag instead of having a
  GetOfflineLinkService method.

License: MIT
Signed-off-by: Steven Allen <steven@stebalien.com>

License: MIT
Signed-off-by: Jeromy <jeromyj@gmail.com>

License: MIT
Signed-off-by: Jeromy <why@ipfs.io>

Also change existing 'Node' type to 'ProtoNode' and use that most
everywhere for now. As we move forward with the integration we will try
and use the Node interface in more places that we're currently using
ProtoNode.

License: MIT
Signed-off-by: Jeromy <why@ipfs.io>

License: MIT
Signed-off-by: Jakub Sztandera <kubuxu@protonmail.ch>