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>