Make `Root` value explicitly a `Directory` structure instead of the `FSNode`
interface (which also allowed the `File` type). This helps to make the code
easier to reason about: the root of an MFS layout is always a directory, not a
(single) file.

Rename `GetValue()` to `GetDirectory()` to also make it more explicit, the
renamed function now returns a `Directory` so there is no need for type
assertions that were previously done on the `FSNode` interface to check that it
was actually a `Directory`.

`NewRoot()` now doesn't allow to create `Root` structures from DAG nodes that
contain UnixFS files.

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