- read-tree: Extract tree from object

This command will take an OID of a tree and extract it to the working directory.
Kind of the opposite of `write-tree`.

I divided the implementation into a few layers:

`_iter_tree_entries` is a generator that will take an OID of a tree, tokenize it
line-by-line and yield the raw string values.

`get_tree` uses `_iter_tree_entries` to recursively parse a tree into a
dictionary.

`read_tree` uses `get_tree` to get the file OIDs and writes them into the
working directory.

Now we can actually save versions of the working directory! It's nothing like
proper version control, but we can see that a super basic flow is possible:

  + Imagine you work on some code and you want to save a version.
  + You run ```ugit write-tree```.
  + You remember that OID that was printed out (write it on a post-it note or
  something :)).
  + Continue working and repeat steps 2 and 3 as necessary.
  + If you want to return to a previous version, use `ugit read-tree` to restore
  it to the working directory.

Is it convenient to use? No. But it's just the beginning!