- write-tree: List files

The next command is `write-tree`. This command will take the current working
directory and store it to the object database. If `hash-object` was for storing
an individual file, then `write-tree` is for storing a whole directory.

Like `hash-object`, `write-tree` is going to give us an OID after it's done and
we'll be able to use the OID in order to retrieve the directory at a later time.

In Git's lingo a "tree" means a directory.

We'll get into the details in later changes, in this change we'll only prepare
the code around the feature:

  + Create a `write-tree` CLI command

  + Create a `write_tree()` function in base module. Why in base module and not
  in data module? Because `write_tree()` is not going to write to disk directly
  but use the object database provided by data to store the directory. Hence it
  belongs to the higher-level base module.

  + Add code to `write_tree()` to print a directory recursively. For now nothing
  is written anywhere, but we just coded the boilerplate to recursively scan a
  directory.

We continue in the next change.